Expandable-collapsible electrode structures made of electrically conductive material

ABSTRACT

Electrode assemblies and associated systems employ a nonporous wall having an exterior for contacting tissue. The exterior peripherally surrounds an interior area. The wall is essentially free of electrically conductive material. The wall is adapted to assume an expanded geometry having a first maximum diameter and a collapsed geometry having a second maximum diameter less than the first maximum diameter. The assemblies and systems include a lumen that conveys a medium containing ions into the interior area. An element free of physical contact with the wall couples the medium within the interior area to a source of electrical energy to enable ionic transport of electrical energy from the source through the medium to the wall for capacitive coupling to tissue contacting the exterior of the wall.

RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 09/300,936, filedApr. 27, 1999, now U.S. Pat. No. 6,179,835, which is a continuation ofapplication Ser. No. 08/628,928, filed Apr. 8, 1996, now U.S. Pat. No.5,925,038, which claims the benefit of U.S. Provisional Pat. App. Ser.Nos. 60/010,223; 60/010,225; and 60/010,354, under 35 U.S.C. 119(e), allof which were filed on Jan. 19, 1996.

FIELD OF THE INVENTION

The invention generally relates to electrode structures deployed ininterior regions of the body. In a more specific sense, the inventionrelates to electrode structures deployable into the heart for diagnosisand treatment of cardiac conditions.

BACKGROUND OF THE INVENTION

The treatment of cardiac arrhythmias requires electrodes capable ofcreating tissue lesions having a diversity of different geometries andcharacteristics, depending upon the particular physiology of thearrhythmia sought to be treated.

For example, a conventional 8 F diameter/4 mm long cardiac ablationelectrode can transmit radio frequency energy to create lesions inmyocardial tissue with a depth of about 0.5 cm and a width of about 10mm, with a lesion volume of up to 0.2 cm³. These small and shallowlesions are desired in the sinus node for sinus node modifications, oralong the AV groove for various accessory pathway ablations, or alongthe slow zone of the tricuspid isthmus for atrial flutter (AFL) or AVnode slow or fast pathway ablations.

However, the elimination of ventricular tachycardia (VT) substrates isthought to require significantly larger and deeper lesions, with apenetration depth greater than 1.5 cm, a width of more than 2.0 cm, anda lesion volume of at least 1 cm³.

There also remains the need to create lesions having relatively largesurface areas with shallow depths.

One proposed solution to the creation of diverse lesion characteristicsis to use different forms of ablation energy. However, technologiessurrounding microwave, laser, ultrasound, and chemical ablation arelargely unproven for this purpose.

The use of active cooling in association with the transmission of DC orradio frequency ablation energy is known to force the tissue interfaceto lower temperature values. As a result, the hottest tissue temperatureregion is shifted deeper into the tissue, which, in turn, shifts theboundary of the tissue rendered nonviable by ablation deeper into thetissue. An electrode that is actively cooled can be used to transmitmore ablation energy into the tissue, compared to the same electrodethat is not actively cooled. However, control of active cooling isrequired to keep maximum tissue temperatures safely below about 100° C.,at which tissue desiccation or tissue boiling is known to occur.

Another proposed solution to the creation of larger lesions, either insurface area and/or depth, is the use of substantially larger electrodesthan those commercially available. Yet, larger electrodes themselvespose problems of size and maneuverability, which weigh against a safeand easy introduction of large electrodes through a vein or artery intothe heart.

A need exists for multi-purpose cardiac ablation electrodes that canselectively create lesions of different geometries and characteristics.Multi-purpose electrodes would possess the flexibility andmaneuverability permitting safe and easy introduction into the heart.Once deployed inside the heart, these electrodes would possess thecapability to emit energy sufficient to create, in a controlled fashion,either large and deep lesions, or small and shallow lesions, or largeand shallow lesions, depending upon the therapy required.

SUMMARY OF THE INVENTION

The invention provides electrode assemblies and associated systemsemploying a nonporous wall having an exterior for contacting tissue. Theexterior peripherally surrounds an interior area. The wall isessentially free of electrically conductive material. The wall isadapted to assume an expanded geometry having a first maximum diameterand a collapsed geometry having a second maximum diameter less than thefirst maximum diameter. The assemblies and systems include a lumen thatconveys a medium containing ions into the interior area. An element freeof physical contact with the wall couples the medium within the interiorarea to a source of electrical energy to enable ionic transport ofelectrical energy from the source through the medium to the wall forcapacitive coupling to tissue contacting the exterior of the wall.

In a preferred embodiment, the capacitive coupling of the wall isexpressed in the following relationship:

{square root over (R_(PATH) ²+L +X_(C) ²+L )}<R_(TISSUE)

where: $R_{PATH} = {\frac{K}{S_{E}}\rho_{s}}$

and

K is a constant that depends upon geometry of the wall,

S_(E) is surface area of the element, and

ρ_(S) is resistivity of the medium containing ions, and

where: $X_{C} = \frac{1}{2\pi \quad {fC}}$

and

f is frequency of the electrical energy, and $C = {ɛ\frac{S_{B}}{t}}$

where:

ε is the dielectric constant of wall,

S_(B) is the area of the interior area, and

t is thickness of the wall located between the medium containing ionsand tissue, and

where R_(TISSUE) is resistivity of tissue contacting the wall.

The invention also provides systems and methods for heating or ablatingbody tissue. The systems and methods provide a catheter tube having adistal end that carries an electrode of the type described above. Thesystems and methods electrically couple a source of radio frequencyenergy to the electrically conductive element within the electrode bodyand to a return electrode in contact with body tissue.

According to this aspect of the invention, the systems and methods guidethe catheter tube into a body with the wall in the collapsed geometryand then cause the wall to assume the expanded geometry at least in partby conveying a medium containing ions into the interior area of thebody. The systems and methods then ohmically heat or ablate body tissueby transmitting radio frequency energy to the electrically conductiveelement for ionic transport through the medium to the wall forcapacitive coupling to tissue located between the return electrode andthe electrode.

Features and advantages of the inventions are set forth in the followingDescription and Drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a system for ablating heart tissue, whichincludes an expandable electrode structure that embodies the features ofthe invention;

FIG. 2 is a side elevation view of an expandable electrode structureusable in association with the system shown in FIG. 1, in which aninflation medium is used to expand the structure;

FIG. 3A is a side elevation view of an alternative expandable electrodestructure usable in association with the system shown in FIG. 1, inwhich an inflation medium is used to expand separate multiple chamberswithin the structure;

FIG. 3B is a side elevation view of an alternative expandable electrodestructure usable in association with the system shown in FIG. 1, inwhich an inflation medium is used to expand integrally formed multiplechambers within the structure;

FIG. 3C is a top section view of the electrode structure shown in FIG.3B, taken generally along line 3C—3C in FIG. 3B;

FIG. 3D is a side elevation view of an alternative expandable electrodestructure usable in association with the system shown in FIG. 1, inwhich an inflation medium is used to expand a single chamber within thestructure;

FIG. 3E is a top view of an alternative expandable-collapsible electrodestructure with a body having interior coextruded webs thatcompartmentalize the body into multiple interior chambers;

FIG. 4 is a side elevation view of an alternative expandable electrodestructure usable in association with the system shown in FIG. 1, inwhich an open spline structure is used to expand the structure;

FIG. 5 is the expandable electrode shown in FIG. 4, in which a slidablesheath is used to collapse the structure;

FIG. 6 is a side elevation view of an alternative expandable electrodestructure usable in association with the system shown in FIG. 1, inwhich an interwoven mesh structure is used to expand the structure;

FIG. 7 is the expandable electrode shown in FIG. 6, in which a slidablesheath is used to collapse the structure;

FIG. 8 is a side elevation view of an alternative expandable interwovenmesh electrode structure usable in association with the system shown inFIG. 1, in which an interior bladder is used to expand the structure;

FIG. 9 is a side elevation view of an alternative expandable foamelectrode structure usable in association with the system shown in FIG.1;

FIG. 10 is a side elevation view of an alternative expandable electrodestructure usable in association with the system shown in FIG. 1, inwhich an electrically actuated spline structure is used to expand thestructure;

FIG. 11A is a side elevation view of an alternative expandable electrodestructure usable in association with the system shown in FIG. 1, inwhich the electrode structure is pleated or creased to promote foldingupon collapse;

FIG. 11B is the electrode shown in FIG. 11A in the process of foldingwhile collapsing;

FIG. 11C is the electrode shown in FIG. 11A as folded upon collapse;

FIG. 12 is a side elevation view of an expandable electrode structureusable in association with the system shown in FIG. 1, in which asteering mechanism proximal to the structure steers the structure at theend of a catheter tube;

FIG. 13 is a side elevation view of an expandable electrode structureusable in association with the system shown in FIG. 1, in which asteering mechanism within the structure steers the structure at the endof a catheter tube;

FIG. 14 is a side elevation view of an expandable electrode structureusable in association with the system shown in FIG. 1, in which anaxially and radially movable stilette in the structure is used to alterthe shape of the structure;

FIGS. 15A to 15E are plan views of an assembly process for manufacturingan expandable electrode structure using an inflation medium to expandthe structure;

FIGS. 16A to 16D are plan views of an assembly process for manufacturingan expandable electrode structure using an interior spline structure toexpand the structure;

FIG. 17 is a side elevation view of an expandable electrode structureusable in association with the system shown in FIG. 1, in which anelectrically conductive shell is deposited on the distal end of thestructure;

FIG. 18 is a side elevation view of an expandable electrode structureusable in association with the system shown in FIG. 1, in which anelectrically conductive foil shell is positioned for attachment on thedistal end of the structure;

FIG. 19 is an enlarged section view of the wall of an expandableelectrode structure usable in association with the system shown in FIG.1, in which an electrically conductive material is coextruded within thewall;

FIG. 20 is a top view of an expandable electrode structure having anexterior shell of electrically conductive material formed in a segmentedbull's-eye pattern;

FIGS. 21 and 22 are, respectively, side and top views of an expandableelectrode structure having an exterior shell of electrically conductivematerial formed in a segmented pattern of energy transmission zonescircumferentially spaced about a preformed, foldable body, and includingmultiple temperature sensing elements;

FIGS. 23, 24A, and 24B are enlarged side views showing the deposition ofelectrically conductive material to establish fold lines on the exteriorof an expandable electrode structure;

FIG. 25 is a top view of an expandable electrode structure showing thepreferred regions for attaching signal wires to an electricallyconductive shell deposited on the distal end of the structure;

FIG. 26 is a side view of an expandable electrode structure showing thepreferred regions for attaching signal wires to an electricallyconductive shell deposited in a circumferentially segmented pattern onthe structure;

FIG. 27 is a top view of an expandable electrode structure showing thepreferred regions for attaching signal wires to an electricallyconductive shell deposited in a bull's-eye pattern on the structure;

FIGS. 28A and 28B are, respectively side section and top views showingthe attachment of signal walls to an electrically conductive shelldeposited on the distal end of the structure, the signal wires being ledthrough the distal end of the structure;

FIG. 29A is an enlarged side view of the distal end of an expandableelectrode structure usable in association with the system shown in FIG.1, showing the attachment of an ablation energy signal wire to theelectrically conductive shell using a mechanical fixture at the distalend of the structure;

FIG. 29B is an enlarged exploded side view, portions of which are insection, of the mechanical fixture shown in FIG. 29A;

FIGS. 30 and 31 are side section views showing the attachment of asignal wire to an electrically conductive shell, the signal wire beingsnaked through the wall of the structure either one (FIG. 30) ormultiple times (FIG. 31);

FIG. 32 is an enlarged section view of the wall of an expandableelectrode structure usable in association with the system shown in FIG.1, showing the laminated structure of the wall and the attachment of anablation energy signal wire to the electrically conductive shell usinglaser windowing techniques;

FIG. 33 is a side view, with portions broken away and in section, of anexpandable electrode structure usable in association with the systemshown in FIG. 1, showing the attachment of a temperature sensing elementto a fixture at the distal end of the structure;

FIG. 34 is an enlarged side section view of the wall of an expandableelectrode structure usable in association with the system shown in FIG.1, showing ways of attaching temperature sensing elements inside andoutside the wall;

FIG. 35 is an enlarged side section view of the wall of an expandableelectrode structure usable in association with the system shown in FIG.1, showing a laminated structure and the creation of temperature sensingthermocouples by laser windowing and deposition;

FIG. 36 is a top view of an expandable electrode structure showing thepreferred regions for attaching temperature sensing elements withrespect to an electrically conductive shell deposited on the distal endof the structure;

FIG. 37 is a side view of an expandable electrode structure showing thepreferred regions for attaching temperature sensing elements withrespect to an electrically conductive shell deposited in acircumferentially segmented pattern on the structure;

FIG. 38 is a top view of an expandable electrode structure showing thepreferred regions for attaching temperature sensing elements withrespect to an electrically conductive shell deposited in a bull's-eyepattern on the structure;

FIG. 39 is a side view of an expandable electrode structure showing apattern of holes for cooling the edge regions of an electricallyconductive shell deposited in a circumferentially segmented pattern onthe structure, the pattern of holes also defining a fold line betweenthe segments of the pattern;

FIGS. 40A and 40B are enlarged views of a hole formed in the structureshown in FIG. 39, showing that the hole defines a fold line;

FIG. 41A is a side sectional view of an expandable electrode structureusable in association with the system shown in FIG. 1, which iscapacitively coupled to tissue;

FIG. 41B is a diagrammatic view showing the electrical path thatablation energy follows when the electrode shown in FIG. 40A iscapacitively coupled to tissue;

FIG. 42A is an side sectional view of an alternative expandableelectrode structure usable in association with the system shown in FIG.1, which is capacitively coupled to tissue;

FIG. 42B is a diagrammatic view showing the electrical path thatablation energy follows when the electrode shown in FIG. 41A iscapacitively coupled to tissue;

FIG. 43 is a diagrammatic view of neural network usable for predictingmaximum temperature conditions when the expandable-collapsible electrodestructure carries multiple ablation energy transmitting segments; and

FIG. 44 is a side elevation view of an expandable electrode structurethat embodies the features of the invention, used in association withpacing and sensing electrodes.

The invention may be embodied in several forms without departing fromits spirit or essential characteristics. The scope of the invention isdefined in the appended claims, rather than in the specific descriptionpreceding them. All embodiments that fall within the meaning and rangeof equivalency of the claims are therefore intended to be embraced bythe claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

I. Overview of a System With an Expandable-Collapsible ElectrodeStructure

FIG. 1 shows a tissue ablation system 10 that embodies the features ofthe invention.

The system 10 includes a flexible catheter tube 12 with a proximal end14 and a distal end 16. The proximal end 14 carries a handle 18. Thedistal end 16 carries an electrode structure 20, which embodies featuresof the invention. The purpose of the electrode structure 20 is totransmit ablation energy.

As the embodiments of FIGS. 2 through 11 show in greater detail, theelectrode structure 20 includes an expandable-collapsible wall forming abody 22. The geometry of the body 22 can be altered between an enlarged,or expanded, geometry having a first maximum diameter (depicted invarious forms, for example, in FIGS. 2, 3, 4, 6, and 11A) and acollapsed geometry having a second maximum diameter less than the firstmaximum diameter (depicted in various forms, for example, in FIGS. 5, 7,11B/C).

This characteristic allows the expandable-collapsible body 22 to assumea collapsed, low profile (ideally, less than 8 French diameter, i.e.,less than about 0.267 cm) when introduced into the vasculature. Oncelocated in the desired position, the expandable-collapsible body 22 canbe urged into a significantly expanded geometry of, for example,approximately 7 to 20 mm.

The expanded geometry of the body 22, coupled with its inherentflexibility, significantly enhances the lesion creation characteristicsof the electrode structure. Further details of the body and the ways toalter its geometry will be provided later.

All or a portion of the wall forming the body 22 carries an electricallyconductive material that forms an electrode surface. As the embodimentsof FIGS. 2 through 11 show in greater detail, the electricallyconductive material comprises an electrically conductive shell 24overlying all or a portion of the expandable-collapsible body 22. Theshell 24 serves as the transmitter of energy that ablates body tissue.While the type of ablation energy used can vary, in the illustrated andpreferred embodiment, the shell 24 serves to transmit radio frequency(RF) electromagnetic energy.

The shell 24 is flexible enough to adopt to the range of geometries,from collapsed to expanded, that the expandable-collapsible body 22assumes. Still, the shell 24 preferably resists stretching within thisrange, to thereby minimize “thinning.” Thinning of the shell 24 createslocalized changes to the shell 24, with attendant increases inresistance and “hot spots.” For this reason, the elasticity of theexpandable-collapsible body 22 and shell 24 should be selected to fallwithin acceptable bounds so that the ability to fold is retained whilepreserving stability during inflation. Further details of the energytransmitting shell 24 will be provided later.

As will be shown in greater detail later (see FIGS. 25 to 32), the shell24 is coupled to one or more signal wires 26. The signal wires 26 extendfrom the shell 24, through the catheter tube 12, to external connectors28 on the handle 18 (see FIG. 1). The connectors 28 electrically couplethe shell 24 to a radio frequency generator 30.

In the preferred and illustrated embodiment (see FIG. 1), a controller32 is associated with the generator 30, either as an integrated unit oras a separate interface box. The controller 32 governs the delivery ofradio frequency ablation energy to the shell 24 according topreestablished criteria. Further details of this aspect of the system 10will be described later.

The system 10 as just described is suited for ablating myocardial tissuewithin the heart. In this environment, a physician moves the cathetertube 12 through a main vein or artery into a heart chamber, while theexpandable-collapsible body 22 of the electrode structure 20 is in itslow profile geometry. Once inside the desired heart chamber, theexpandable-collapsible body 22 is enlarged into its expanded geometry,and the shell 24 is placed into contact with the targeted region ofendocardial tissue. Radio frequency energy is conveyed from thegenerator 30 to the shell 24, as governed by the controller 32. Theshell 24 transmits radio frequency energy into tissue to a returnelectrode, which is typically an external patch electrode (forming aunipolar arrangement). Alternatively, the transmitted energy can passthrough tissue to an adjacent electrode in the heart chamber (forming abipolar arrangement), or between segments in the shell 24, as will bedescribed later (also forming a bipolar arrangement). The radiofrequency energy heats the tissue forming a lesion.

The expanded geometry of the expandable-collapsible body 22 enhances theenergy transmission characteristics of the structure 20. The structure20, when expanded, is able to form tissue lesions that are significantlylarger in terms of size and volume than the body's initial collapsedprofile during introduction would otherwise provide.

It should also be appreciated that the expandable-collapsible electrodestructure 20 as just described is also suited for mapping myocardialtissue within the heart. In this use, the shell 24 senses electricalactivity in the heart. The sensed electrical activity is conveyed to anexternal monitor, which processes the potentials for analysis by thephysician. The use of an expandable-collapsible electrode structure forthis purpose is generally disclosed in Edwards et al. U.S. Pat. No.5,293,869.

It should also be appreciated that the expandable-collapsible electrodestructure 20 can be used alternatively, or in combination with sensingelectrical activities, to convey pacing signals. In this way, thestructure 20 can carry out pace mapping or entrainment mapping. Theexpanded electrode structure 20 can also be used to convey pacingsignals to confirming contact with tissue before ablating. The abilityto carry out pacing to sense tissue contact is unexpected, given thatthe expanded structure 20 presents a surface area significantly greaterthan that presented by a conventional 4 mm/8 F electrode.

As FIG. 44 shows, the catheter tube 20 can also carry one or moreconventional ring electrodes 21 for bipolar sensing. A conventionalpacing or unipolar sensing electrode 23 may also be provided, appendedat the distal end of the structure 20.

II. The Expandable-Collapsible Body

The expandable-collapsible body 22 is made from a material selected toexhibit the following characteristics:

(i) the material must be capable, in use, of transition between anexpanded geometry having a first maximum diameter and a collapsedgeometry having a second maximum diameter less than the first diameter.In this respect, the material can be formed into anexpandable-collapsible bladder or balloon body having an open interior.The body is flexible enough to assume the expanded geometry as a resultof a normally open solid support structure within the interior, or theopening of a normally closed support structure within the interior, orthe introduction of fluid pressure into the interior, or a combinationof such interior forces. In this arrangement, the body is caused toassume the collapsed geometry by an exterior compression force againstthe normally open interior support structure, or the closing of theinterior support structure, or the removal of the interior fluidpressure, or a combination of such offsetting forces. Alternatively, thematerial can be a preformed body with a memory urging it toward anormally expanded geometry. In this arrangement, the preformed body iscaused to assume the collapsed geometry by the application of anexternal compression force. In this arrangement, the preformed body canhave an open interior, or can comprise, for example, a collapsiblecomposite foam structure.

(ii) the material must be biocompatible and able to withstand hightemperature conditions, which arise during manufacture and use.

(iii) the material must possess sufficient strength to withstand,without rupture or tearing, external mechanical or fluid forces, whichare applied to support and maintain its preformed geometry during use.

(iv) the material must lend itself to attachment to the catheter tube 12through the use of straightforward and inexpensive adhesive, thermal, ormechanical attachment methods.

(v) the material must be compatible with the electrically conductiveshell 24 to achieve secure adherence between the two.

Thermoplastic or elastomeric materials that can be made to meet thesecriteria include polyimide. (kapton), polyester, silicone rubber, nylon,mylar, polyethelene, polyvinyl chloride, and composite structures usingthese and other materials.

The incidence of tissue sticking to the exterior of the body 22 duringuse can be mediated by the inclusion of low friction materials likePTFE. The propensity of the exterior of the body 22 to cause bloodclotting and/or embolization can be reduced by incorporatingnon-thrombogenic material onto or into the exterior of the body 22.

Polyimide is particularly preferred for the expandable-collapsible body.Polyimide is flexible, but it is not elastic. It can withstand very hightemperatures without deformation. Because polyimide is not elastic, itdoes not impose stretching forces to the shell, which could lead toelectrical conductivity decreases, as above described.

The expandable-collapsible body 22 can be formed about the exterior of aglass mold. In this arrangement, the external dimensions of the moldmatch the desired expanded internal geometry of theexpandable-collapsible body 22. The mold is dipped in a desired sequenceinto a solution of the body material until the desired wall thickness isachieved. The mold is then etched away, leaving the formedexpandable-collapsible body 22.

Various specific geometries, of course, can be selected. The preferredgeometry is essentially spherical and symmetric, with a distal sphericalcontour, as FIGS. 2 to 11 show in various forms. However, nonsymmetricgeometries can be used. For example, the expandable-collapsible body 22may be formed with a flattened distal contour, which gradually curves ornecks inwardly for attachment with the catheter tube 12.

The expandable-collapsible body 22 may also be blow molded from anextruded tube. In this arrangement, the body 22 is sealed at one endusing a mechanical clamp, adhesive, or thermal fusion. The opposite openend of the body 22 is left open. The sealed expandable-collapsible body22 is placed inside the mold. An inflation medium, such as high pressuregas or liquid, is introduced through the open tube end. The mold isexposed to heat as the tube body 22 is inflated to assume the moldgeometry. The formed expandable-collapsible body 22 is then pulled fromthe mold.

A. Expansion Using Interior Fluid Pressure

In the embodiments shown in FIGS. 2 and 3A/B/C, fluid pressure is usedto inflate and maintain the expandable-collapsible body 22 in theexpanded geometry.

In this arrangement, the catheter tube 12 carries an interior lumen 34along its length. The distal end of the lumen 34 opens into the hollowinterior of the expandable-collapsible body 22, which has been formed inthe manner just described. The proximal end of the lumen 34 communicateswith a port 36 (see FIG. 1) on the handle 18.

An inflation fluid medium (arrows 38 in FIG. 2) is conveyed underpositive pressure through the port 36 and into the lumen 34. The fluidmedium 38 fills the interior of the expandable-collapsible body 22. Thefluid medium 38 exerts interior pressure to urge theexpandable-collapsible body 22 from its collapsed geometry to theenlarged geometry desired for ablation.

The inflating fluid medium 38 can vary. Preferably, it comprises aliquid such as water, saline solution, or other biocompatible fluid.Alternatively, the inflating fluid medium 38 can comprise a gaseousmedium such as carbon dioxide or air.

Regardless of the type of fluid medium 38, the inflation preferablyoccurs under relatively low pressures of up to 30 psi. The pressure useddepends upon the desired amount of inflation, the strength and materialused for the body 22, and the degree of flexibility required, i.e., highpressure leads to a harder, less flexible body 22.

More than one fluid conveying lumen 34 may be used. The multiple lumens34 can, for example, speed up the introduction or removal of theinflating medium 38 from the body 22. Multiple lumens can also serve tocontinuously or intermittently recycle the inflating medium 38 withinthe body 22 for controlling the temperature of the body, as will bedescribed in greater detail later. Multiple lumens can also be used,with at least one of the lumens dedicated to venting air from thestructure 20.

In an alternative embodiment shown in FIG. 3A, a group of sealedbladders compartmentalize the interior of the formed body into chambers40. One or more lumens 42 passing through the catheter tube 12 conveythe inflating gas or liquid medium 38 into each chamber 40, as describedabove. The inflated chambers 40 collectively hold theexpandable-collapsible body 22 in its expanded condition. Removal of theinflation medium 38 deflates the chambers 40, collapsing theexpandable-collapsible body 22.

The bladders defining the chambers 40 may be separately formed bymolding in generally the same fashion as the main expandable-collapsiblebody 22. The bladder material need not have the same resistance to hightemperature deformation as the expandable-collapsible body 22. Ifdesired, the bladders may also be deposition coated with a thermalinsulating material to thermally insulate them from the mainexpandable-collapsible body 22.

Alternatively, as FIGS. 3B and 3C show, the interior chambers 40 cantake the form of tubular, circumferentially spaced ribs 41 attached tothe interior of the body 22. In this arrangement, the ribs 41 preferablyconstitute integrally molded parts of the body 22.

As explained in connection with the FIG. 3A embodiment, a single lumenmay service all chambers ribs 41. However, multiple lumens individuallycommunicating with each rib 41 provide the ability to more particularlycontrol the geometry of the expanded body 22, by selectively inflatingsome but not all the ribs 41 or chambers 40.

As FIG. 3E shows, the body 22 may be extruded with interior webs 43.When the body is in its expanded geometry, the interior webs 43compartmentalize the body 22 into the interior chambers 40, as alreadydescribed. As before described, multiple lumens preferably individuallycommunicate with each formed chamber 40 for conveying inflation mediumand for venting air.

As FIG. 3D shows, a separate, single interior chamber 124 can be usedinstead of the compartmentalized chambers 40 or ribs 41 shown in FIGS.3A, 3B, and 3C to receive the inflation medium for the exterior body 22.As will be described in greater detail later, this arrangement createsan intermediate region 126 between the interior of the body 22 and theexterior of the chamber 124, through which signal wires 26 can be passedfor coupling to the shell 24.

B. Interior Support Structures

In the embodiments shown in FIGS. 4 to 7, collapsible, interiorstructures 44 sustain the expandable-collapsible body 22 in the expandedgeometry. The presence of the interior support structure 44 eliminatesthe need to introduce air or liquid as an inflation medium 38. Possibledifficulties of fluid handling and leakage are thereby avoided.

In the embodiment shown in FIGS. 4 and 5, the expandable-collapsiblebody 22 is held in its expanded geometry by an open interior structure44 formed by an assemblage of flexible spline elements 46. The splineelements 46 are made from a resilient, inert wire, like nickel titanium(commercially available as Nitinol material), or from a resilientinjection molded inert plastic or stainless steel. The spline elements46 are preformed in a desired contour and assembled to form a threedimensional support skeleton, which fills the interior space of theexpandable-collapsible body 22.

In this arrangement, the supported expandable-collapsible body 22 isbrought to a collapsed geometry by outside compression applied by anouter sheath 48 (see FIG. 5), which slides along the catheter tube 12.As FIG. 5 shows, forward movement of the sheath 48 advances it over theexpanded expandable-collapsible body 22. The sliding sheath 48encompasses the expandable-collapsible body 22, compressing the interiorspline elements 46 together. The expandable-collapsible body 22collapses into its low profile geometry within the sheath 48.

Rearward movement of the sheath 48 (see FIG. 4) retracts it away fromthe expandable-collapsible body 22. Free from the confines of the sheath48, the interior support structure 44 of spline elements 46 springs openinto the three dimensional shape. The expandable-collapsible body 22returns to its expanded geometry upon the spline elements 46.

In an alternative embodiment, as FIGS. 6 and 7 show, theexpandable-collapsible body 22 is supported upon a closed, threedimensional structure 44 formed by a resilient mesh 50. The meshstructure 50 is made from interwoven resilient, inert wire or plasticfilaments preformed to the desired expanded geometry. The mesh structure50 provides interior support to hold the expandable-collapsible body 22in its expanded geometry, in the same way as the open structure ofspline elements 46 shown in FIG. 4.

As FIG. 7 further shows, a sliding sheath 48 (as previously described)can also be advanced along the catheter tube 12 to compress the meshstructure 50 to collapse mesh structure 50 and, with it, theexpandable-collapsible body 22. Likewise, retraction of the sheath 48removes the compression force (as FIG. 6 shows), and the freed meshstructure 50 springs open to return the expandable-collapsible body 22back to its expanded geometry.

By interweaving the mesh filaments close enough together, the meshstructure 50 itself could serve as the support for the electricallyconductive shell 24, without need for the intermediateexpandable-collapsible body 22. Indeed, all or a portion of the meshfilaments could be made electrically conductive to themselves serve astransmitters of ablation energy. This arrangement of interwoven,electrically conductive filaments could supplement or take the place ofthe electrically conductive shell 24.

Alternatively, as FIG. 8 shows, the mesh structure 50 can be made tonormally assume the collapsed geometry. In this arrangement, one or moreinterior bladders 126 can accommodate the introduction of an inflationmedium to cause the mesh structure 50 to assume the expanded geometry.

If the mesh structure 50 is tightly woven enough to be essentiallyliquid impermeable , the interior bladder 126 could be eliminated. Inthis arrangement, the introduction of a biocompatible liquid, such assterile saline, directly into the interior of the structure 50 wouldcause the structure to assume the expanded geometry.

FIG. 9 shows yet another alternative expandable-collapsible structure.In this embodiment, a foam body 128 molded to normally assume the shapeof the expanded geometry forms the interior support structure for thebody 22. As with the interior structures 44, the presence of the foambody 128 eliminates the need to introduce air or liquid as an inflationmedium. Also like the interior structures 44, a sliding sheath (notshown but as previously described) can be advanced along the cathetertube 12 to compress the foam body 128 and overlying body 22 into thecollapsed geometry. Likewise, retraction of the sheath removes thecompression force. The foam body 128, free of the sheath, springs opento return the expandable-collapsible body 22 back to the expandedgeometry. It should be appreciated that the foam body 128 can provideinterior, normally expanded support to the mesh structure 50 in the sameway.

As FIG. 10 shows, the geometry of the expandable-collapsible body 22 canbe controlled electrically. This arrangement includes an assemblage ofspline elements 132 within the body 22. The spline elements 132 are madeof a material that undergoes shape or phase change in response toheating. Nickel titanium wire is a material having this characteristic.Alternatively, the spline elements 132 could comprise an assembly of twometals having different coefficients of expansion.

The body 22 overlies the spline elements 132. The spline elements 132are coupled to an electrical current source 134. Current flow from thesource 134 through the spline elements 132 resistively heats theelements 132. As a result, the spline elements 132 change shape.

As FIG. 10 shows, the spline elements 132 normally present the collapsedgeometry. Current flow through the spline elements 132 causes expansionof the elements 132, thereby creating the expanded geometry (as shown byarrows and phantom lines in FIG. 10). It should be appreciated that thespline elements 132 could alternatively normally present the expandedgeometry and be made to contract, thereby assuming the collapsedgeometry, in response to current flow.

C. Folding

In all the representative embodiments, the expandable-collapsible body22 can be molded with preformed regions 52 (see FIGS. 11A/B/C) ofreduced thickness, forming creases. To create these crease regions 52,the mold has a preformed surface geometry such that theexpandable-collapsible material would be formed slightly thinner,indented, or ribbed along the desired regions 52. Alternatively, the useof interior coextruded webs 43, as FIG. 3E shows, also serves to formthe crease regions 52 along the area where the webs 43 contact theinterior wall of the body 22.

As FIGS. 11B/C show, the expandable-collapsible body 22 collapses aboutthese regions 52, causing the body 22 to circumferentially fold uponitself in a consistent, uniform fashion. The resulting collapsedgeometry can thus be made more uniform and compact.

In the embodiments where an inflation medium 38 applies positivepressure to expand the expandable-collapsible body 22, a negative fluidpressure can be applied inside the expandable-collapsible body 22 todraw the fold regions 52 further inward. In the embodiment where theinterior structure 44 of open spline elements 46 supports theexpandable-collapsible body 22, the fold regions 52 are preferablyaligned in the spaces between the spline elements 46 to take bestadvantage of the prearranged folding action.

Alternative ways of creating fold regions 52 in the body 22 will bedescribed in greater detail later.

D. Steering

In the illustrated and preferred embodiment, a distal steering mechanism54 (see FIG. 1) enhances the manipulation of the electrode structure 20,both during and after deployment.

The steering mechanism 54 can vary. In the illustrated embodiment (seeFIG. 1), the steering mechanism 54 includes a rotating cam wheel 56coupled to an external steering lever 58 carried by the handle 18. Thecam wheel 56 holds the proximal ends of right and left steering wires60. The wires 60 pass with the ablation energy signal wires 26 throughthe catheter tube 12 and connect to the left and right sides of aresilient bendable wire or leaf spring 62 adjacent the distal tube end16 (see FIG. 12). Further details of this and other types of steeringmechanisms are shown in Lundquist and Thompson U.S. Pat. No. 5,254,088,which is incorporated into this Specification by reference.

In FIG. 12, the leaf spring 62 is carried within in the distal end 16 ofthe catheter tube 12, to which the electrode structure 20 is attached.As FIGS. 1 and 12 show, forward movement of the steering lever 58 pullson one steering wire 60 to flex or curve the leaf spring 62, and, withit, the distal catheter end 16 and the electrode structure 20, in onedirection. Rearward movement of the steering lever 58 pulls on the othersteering wire 60 to flex or curve the leaf spring 62, and, with it, thedistal catheter end 16 and the electrode structure 20, in the oppositedirection.

In FIG. 13, the leaf spring 62 is part of a distal fixture 66 carriedwithin the electrode structure 20 itself. In this arrangement, the leafspring 62 extends beyond the distal catheter end 16 within a tube 64inside the expandable-collapsible body 22. The distal end of the leafspring 62 is secured to a distal fixture 66. The distal fixture 66 isitself attached to the distal end of the body 22. Further details ofattaching the fixture 66 to the distal end of the body 22 will bedescribed in greater detail later.

As FIG. 13 shows, forward movement of the steering lever 58 bends theleaf spring 62 in one direction within the expandable-collapsible body22, deflecting the distal fixture 66 with it. This deforms theexpandable-collapsible body 22 in the direction that the leaf spring 62bends. Rearward movement of the steering lever 58 bends the leaf spring62 in the opposite direction, having the opposite deformation effectupon the expandable-collapsible body 22.

In either arrangement, the steering mechanism 54 is usable whether theexpandable-collapsible body is in its collapsed geometry or in itsexpanded geometry.

E. Push-Pull Stiletto

In FIG. 14, a stilette 76 is attached to the distal fixture 66. Thestilette extends inside the body 22, through the catheter tube 12, to asuitable push-pull controller 70 on the handle 18 (see FIG. 1). Thestilette 76 is movable along the axis of the catheter tube 12. Movingthe stilette 76 forward pushes axially upon the distal fixture 66.Moving the stilette 76 rearward pulls axially upon the distal fixture66. The geometry of the body 22 elongates or expands accordingly.

The stilette 76 can be used in association with anexpandable-collapsible body 22 that is expanded by an inflation medium38. In this arrangement, when the expandable-collapsible body 22 iscollapsed, forward movement of the stilette 76, extends the distalfixture 66 to further urge the expandable-collapsible body 22 into asmaller diameter profile for introduction.

When used in association with an expandable-collapsible body 22 that isinternally supported by the spline structure 46 or the mesh structure50, the stilette 76 can be used instead of the slidable outer sheath 48to expand and collapse the expandable-collapsible body 22. Pushingforward upon the stilette 76 extends the spline structure 46 or meshstructure 50 to collapse the expandable-collapsible body 22. Pullingrearward upon the stilette 76, or merely releasing the pushing force,has the opposite effect, allowing the spline structure 46 or meshstructure 50 to assume its expanded geometry.

When used with either inflated or mechanically expandedexpandable-collapsible bodies 22, pulling rearward upon the stilette 76also has the effect of altering the expanded geometry by flattening thedistal region of the expandable-collapsible body 22.

While the stilette 76 can be used by itself, in the illustratedembodiment (see FIG. 14), the distal end of the stilette 76 near thefixture 66 comprises the bendable leaf spring 62, thereby providing aradial steering function in tandem with the axial push-pull action ofthe stilette 76.

There are various ways to combine the steering mechanism 54 with thestilette 76. In the illustrated embodiment, a collar 136 is retained bya heat-shrink fit within tubing 64. The collar 136 has a centralaperture 138 through which a leaf spring 62 at the end of the stilette76 passes for movement along the axis of the catheter tube 12. Steeringwires 60 are attached to the collar 136. Pulling on the steering wires60 radially deflects the collar 136, thereby bending the leaf spring 62at the end of the stilette 76 in the direction of the pulled steeringwire 60.

F. Attachment to Catheter Tube

A sleeve 78 (see, e.g., FIG. 2) couples the near end of theexpandable-collapsible body 22 to the distal end 16 of the cathetertube. The sleeve 78 withstands the forces exerted to expand theexpandable-collapsible body 22, resisting separation of the body 22 fromthe catheter tube 12. In FIG. 2, where an inflation medium 38 is used,the sleeve 78 also forms a fluid seal that resists leakage of the mediumat inflation pressures.

The sleeve 78 can be secured about the catheter tube in various ways,including adhesive bonding, thermal bonding, mechanical bonding, screws,winding, or a combination of any of these.

FIGS. 15A to 15E show the details of a preferred assembly process for anexpandable-collapsible body 22 whose geometry is altered by use of fluidpressure, such as previously shown in FIGS. 2 and 3. The body 22 isextruded as a tube 140 having an extruded interior diameter, designatedID₁ (see FIG. 15A) . The extruded interior diameter ID₁ is selected tobe less than the exterior diameter of the distal stem 142 of thecatheter tube 12 to which the body 22 will ultimately be attached.

As FIG. 15D shows, the stem 142 comprises an elongated, stepped-downtubular appendage, which extends beyond the distal end 16 of thecatheter tube 12. The distal end of the stem 142 is sealed. The exteriordiameter of the stem 142 is designated in FIG. 15D as ED_(S). The stem142 includes a central lumen 152 for carrying inflation medium. Spacedapart holes 154 on the stem 142 communicate with the lumen to convey theinflation medium into the body 22, when attached to the stem 142.

As FIG. 15A shows, the material of the extruded tube 140 is preferablycross linked by exposure to gamma radiation 168 or an equivalentconventional treatment. The cross linking enhances the capability of thematerial of the tube 140 to recover its shape after mechanicaldeformation.

After cross linking, the extruded tube 140 is mechanically deformed byheat molding into the body 22 having the desired collapsed geometry, ina manner previously described. The body geometry (see FIG. 15B) includesproximal and distal neck regions 144 and 146 and an intermediate mainbody region 148. The neck regions 144 and 146 have an enlarged interiordiameter (designated ID₂ in FIG. 15B) that is slightly greater thancatheter stem diameter ED_(S), to permit a slip fit of the body 22 overthe stem 142. The intermediate main body region 148 has an enlargedexterior diameter selected for the collapsed geometry of the body 22. Topreserve the desired wall thickness, the enlarged exterior diameter ofthe tube 140 should be about twice the original extruded outer diameterof the tube 140.

As FIG. 15C shows, the tubing ends 150 extending beyond the neck regions144 and 146 are cut away. As FIG. 15D shows, the body 22 is slip fittedover the stem 142. Heat is applied to shrink fit the neck regions 144and 146 about the stem 142 (see FIG. 15E). Due to molding, the memory ofthese regions 144 and 146, when heated, seek the original interiordiameter ID₁ of the tubing 140, thereby proving a secure interferencefit about the stem 142.

Preferably, after forming the interference fit between the neck regions144 and 146 and the stem 142, additional heat is provided to thermallyfuse the regions 144 and 146 to the stem 142. Last, the sleeve 78 isheat-shrunk in place about the proximal neck region 144 (see FIG. 15E).The sleeve 78 can comprise a heat-shrink plastic material or phasechangeable metal material, like nickel titanium. Alternatively, thesleeve 78 can be heat-shrunk into place without an intermediate thermalfusing step.

FIGS. 16A to 16D show the details of a preferred assembly process for anexpandable-collapsible body 22 whose geometry is altered by use of aninterior support structure 44 of spline elements 46, such as previouslyshown in FIGS. 4 and 5. After heat molding the body 22 in the mannershown in FIGS. 15A to 15C, the distal neck region 146 is secured by heatshrinking about the distal fixture 66 (see FIG. 16A). As FIG. 16A shows,the distal fixture 66 has, preattached to it, the distal end of thespline element structure 44, as well as any desired steering mechanism54, stilette 76, or combination thereof (not shown in FIGS. 16A to 16D).When initially secured to the fixture 66, the main region 148 of thebody 22 is oriented in a direction opposite to the spline elementstructure 44.

After securing the distal neck region 146 to the fixture 66, as justdescribed, the body 22 is everted about the distal fixture 66 over thespline element structure 44 (see FIG. 16B). The proximal end of thespline element structure 44 is secured to an anchor 156 carried by thedistal catheter end 16 (see FIG. 16C), and the everted proximal neckregion 144 is then slip fitted over the catheter stem 158. As FIG. 16Cshows, the catheter stem 158 in this arrangement does not extend beyondthe neck region 144 of the body 22.

Heat is then applied to shrink fit the neck region 144 about the stem158 (see FIG. 16D). Preferably, after forming this interference fitbetween the neck region 144 and the stem 158, additional heat isprovided to thermally fuse the region 144 to the stem 158. Last, thesleeve 78 is heat-shrunk in place about the proximal neck region 144.Alternatively, the sleeve 78 can be heat-shrunk into place without anintermediate thermal fusing step.

III. The Electrically conducting Shell

The purpose of the electrically conducting shell 24 is to transmitablation energy, which in the illustrated and preferred embodimentcomprises electromagnetic radio frequency energy with a frequency belowabout 1.0 GHz. This type of ablating energy heats tissue, mostlyohmically, to form lesions without electrically stimulating it. In thisarrangement, the shell 24 should possess the characteristics of bothhigh electrical conductivity and high thermal conductivity. It shouldalso be appreciated that the shell 24 could form an antenna for thetransmission of higher frequency microwave energy.

By altering the size, location, and pattern of the shell 24, along withadjusting the power level and time that the radio frequency ablationenergy is transmitted, the electrode structure 20 is able to createlesions of different size and geometries.

A. Shell Geometry (Thermal Convective Cooling)

In one application, the shell creates lesion patterns greater than about1.5 cm deep and/or about 2.0 cm wide. These lesion patterns aresignificantly deeper and wider than those created by conventional 8 Fdiameter/4 mm long electrodes, which are approximately 0.5 cm deep and10 mm wide. The deeper and wider lesion patterns that the shell 24 canprovide are able to destroy epicardial and intramural ventriculartachycardia (VT) substrates.

As the following Example shows, the size and location of the shell 24 onthe expandable-collapsible body 22, when expanded, significantly affectsthe size and geometry of the lesions formed by transmitting radiofrequency ablation energy.

EXAMPLE 1

Finite element analysis was performed for a flexible, expanded electrodestructure 20 having a 1.4 cm diameter and a wall thickness ofapproximately 200 μm. The model assumed a 100 μm thick coating of goldover the distal hemisphere of the structure 20, forming the electricallyconductive shell 24. The constraint for the model was a lower limit onthickness and therefore the thermal conductivity of the shell 24.

For the model, the percent of electrically conductive shell 24 incontact with myocardial tissue, with the balance exposed to blood, waschanged from 5%, 20%, 41%, and 100% tissue contact. Time and power ofenergy transmission were also varied. Power was changed to keep themaximum temperature of tissue under the shell 24 at 90° C. Maximumlesion depth, width, and volume were measured.

The following Table 1 presents the results:

TABLE 1 LESION GEOMETRY AS A FUNCTION OF TISSUE vs. BLOOD CONTACT WITHTHE ELECTRICALLY CONDUCTIVE SHELL % Tis- sue Lesion Lesion Lesion Con-Temp. Voltage Current Power Depth Width Volume tact (° C.) (Volts)(Amps) (Watts) (cm) (cm) (cm²) <5% 92.1 84 1.67 140 2.1 5.4 36 20% 89.781 1.55 125 2.5 4.9 41 41% 89.6 77 1.4 107 2.3 3.5 17 100% 92.3 61 0.9256 1.4 2.6 7

The lesions created in the above Table 1 are capable of makingtransmural lesions in the left ventricle and can therefore ablateepicardial VT substrates. The Table 1 shows that lesion size increaseswith an electrically conductive shell 24 presenting less percentagecontact with tissue than blood. The shell presenting 100% contact withtissue (and none with blood), compared to the shell 24 presenting up to41% percent of its surface to tissue had lower lesion depths.

With less relative contact with tissue than blood, the shell 24 is moreexposed to the blood pool and its convective cooling effect. The bloodcools the shell 24 it contacts. Heat is lost from tissue under the shell24 into the blood pool. This emulation of active cooling of the shell 24causes more power to be transmitted to the tissue before maximum tissuetemperatures are achieved, thereby creating larger lesions.

Table 1 highlights the importance of relatively high thermalconductivity for the shell 24, which can be achieved by materialselection and controlling thickness. Given the same percentage contactwith tissue versus blood, a higher thermal conductivity results in ahigher cooling effect and a corresponding increase in lesion size.

The above Table 1 demonstrates the ability of the structure 20 carryingthe shell 24 to transmit the proper amount of radio frequency energy tocreate large and deep lesions.

Additional tests were performed using shells 24 with a desirable lowerpercentage contact with tissue relative to blood (less than 50%). Thesetests varied the time of ablation energy transmission to gauge theeffect upon lesion size.

The following Table 2 presents the results:

TABLE 2 LESION GEOMETRY AS A FUNCTION OF TIME OF ABLATION ENERGYTRANSMISSION, GIVEN THE SAME TISSUE vs. BLOOD CONTACT WITH THEELECTRICALLY CONDUCTIVE SHELL % Tissue Power Time Lesion Lesion Contact(Watts) (Sec.) Depth (cm) Width (cm) 5% 110 25 0.5 1.6 5% 110 60 1.2 2.441% 67 25 0.35 1.4 41% 67 60 0.9 2.0

The above Table 2 demonstrates the ability of the structure carrying theshell 24 to transmit the proper amount of radio frequency energy tocreate wide and shallow lesions. The effect is achieved by controllingboth the delivered radio frequency power and the time of radio frequencyenergy application. Wide and shallow lesion patterns are effective inthe treatment of some endocardially located substrates and atrialfibrillation substrates.

Tables 1 and 2 demonstrate the capability of the sameexpandable-collapsible electrode structure 20 with the desirable lowerpercentage contact with tissue relative to blood (less than 50%) toablate epicardial, intramural, or endocardial substrates with a range oflesion patterns from wide and shallow to large and deep.

B. Surface Deposition of Shell

The electrically conductive shell 24 may be deposited upon the exteriorof the formed expandable-collapsible body 22.

In this embodiment, a mask is placed upon the surface of theexpandable-collapsible body 22 that is to be free of the shell 24.Preferably, as generally shown in FIG. 17, the shell 24 is not depositedon at least the proximal ⅓rd surface of the expandable-collapsible body22. This requires that at least the proximal ⅓rd surface of theexpandable-collapsible body 22 be masked, so that no electricallyconductive material is deposited there.

The masking of the at least proximal ⅓rd surface of theexpandable-collapsible body 22 is desirable for several reasons. Thisregion is not normally in contact with tissue, so the presence ofelectrically conductive material serves no purpose. Furthermore, thisregion also presents the smallest diameter. If electrically conductive,this region would possess the greatest current density, which is notdesirable. Masking the proximal region of smallest diameter, which isusually free of tissue contact, assures that the maximum current densitywill be distributed at or near the distal region of theexpandable-collapsible body 22, which will be in tissue contact. Thepresence of the steering mechanism 54, already described, also aids inplacing the shell-carrying distal tip in tissue contact.

The shell 24 comprises a material having a relatively high electricalconductivity, as well as a relative high thermal conductivity. Materialspossessing these characteristics include gold, platinum,platinum/iridium, among others. These materials are preferably depositedupon the unmasked, distal region of the expandable-collapsible body 22.Usable deposition processes include sputtering, vapor deposition, ionbeam deposition, electroplating over a deposited seed layer, or acombination of these processes.

Preferably (see FIG. 17), to enhance adherence between theexpandable-collapsible body 22 and the shell 24, an undercoating 80 isfirst deposited on the unmasked distal region before depositing theshell 24. Materials well suited for the undercoating 80 includetitanium, iridium, and nickel, or combinations or alloys thereof.

The total thickness of the shell 24 deposition, including theundercoating 80, can vary. Increasing the thickness increases thecurrent-carrying and thermal conductive capacity of the shell 24.However, increasing the thickness also increases the potential of shellcracking or peeling during enlargement or collapse of the underlyingexpandable-collapsible body 22.

In a preferred embodiment, the deposition of the electrically conductiveshell material should normally have a thickness of between about 5 μmand about 50 μm. The deposition of the adherence undercoating 80 shouldnormally have a thickness of about 1 μm to about 5 μm.

C. Foil Shell Surface

In an alternative embodiment (see FIG. 18), the shell 24 comprises athin sheet or foil 82 of electrically conductive metal affixed to thewall of the expandable-collapsible body 22. Materials suitable for thefoil include platinum, platinum/iridium, stainless steel, gold, orcombinations or alloys of these materials. The foil 82 is shaped into apredetermined geometry matching the geometry of theexpandable-collapsible body 22, when expanded, where the foil 82 is tobe affixed. The geometry of the metal foil 82 can be accomplished usingcold forming or deep drawing techniques. The foil 82 preferably has athickness of less than about 0.005 cm (50 μm). The foil 82 is affixed tothe expandable-collapsible body 22 using an electrically insulatingepoxy, adhesive, or the like.

The shell 24 of foil 82 offers advantages over the deposited shell 24.For example, adherence of the shell foil 82 upon theexpandable-collapsible body 22 can be achieved without using thedeposited undercoating 80. The shell foil 82 also aids in the directconnection of ablation energy wires 26, without the use of additionalconnection pads and the like, as will be described in greater detaillater. The shell foil 82 also offers greater resistance to stretchingand cracking in response to expansion and collapse of the underlyingexpandable-collapsible body 22. This offers greater control overresistance levels along the ablation energy transmitting surface.

D. Co-Extruded Electrically Conductive Shell

In an alternative embodiment (see FIG. 19), all or a portion of theexpandable-collapsible wall forming the body 22 is extruded with anelectrically conductive material 84. Materials 84 suitable forcoextrusion with the expandable-collapsible body 22 include carbon blackand chopped carbon fiber. In this arrangement, the coextrudedexpandable-collapsible body 22 is itself electrically conductive. Anadditional shell 24 of electrically conductive material can beelectrically coupled to the coextruded body 22, to obtain the desiredelectrical and thermal conductive characteristics. The extra externalshell 24 can be eliminated, if the coextruded body 22 itself possessesthe desired electrical and thermal conductive characteristics.

The integral electrically conducting material 84 coextruded into thebody 22 offers certain advantages over the external deposited shell 24(FIG. 17) or shell foil 82 (FIG. 18). Coextrusion avoids the necessityof adherence between the shell 24 and the expandable-collapsible body22. A body 22 coextruded with electrically conducting material 84 alsopermits more direct connection of ablation energy wires 34, without theuse of additional connection pads and the like. The integrated nature ofthe coextruded material 84 in the body 22 protects against cracking ofthe ablation energy transmitting surface during expansion and collapseof the expandable-collapsible body 22.

The integral electrically conducting material 84 coextruded into thebody 22 also permits the creation of a family of electrode structures20, with the structures 20 differing in the amount of conductivematerial 84 coextruded into the wall of the respective body 22. Theamount of electrically conductive material coextruded into a given body22 affects the electrical conductivity, and thus the electricalresistivity of the body 22, which varies inversely with conductivity.Addition of more electrically conductive material increases electricalconductivity of the body 22, thereby reducing electrical resistivity ofthe body 22, and vice versa. It is thereby possible to specify among thefamily of structures 20 having electrically conductive bodies 22, theuse of a given structure 20 according to a function that correlatesdesired lesion characteristics with the electrical resistivity values ofthe associated body 22.

EXAMPLE 2

A three-dimensional finite element model was created for an electrodestructure having a body with an elongated shape, with a total length of28.4 mm, a diameter of 6.4 mm, and a body wall thickness of 0.1 mm. Thebody of the structure was modeled as an electric conductor. Firm contactwith cardiac tissue was assumed along the entire length of the electrodebody lying in a plane beneath the electrode. Contact with blood wasassumed along the entire length of the electrode body lying in a planeabove the electrode. The blood and tissue regions had resistivities of150 and 500 ohm.cm, respectively.

Analyses were made based upon resistivities of 1.2 k-ohm.cm and 12k-ohm.cm for the electrode body.

Table 3 shows the depth of the maximum tissue temperature when RFablation power is applied to the electrode at various power levels andat various levels of resistivity for the body of the electrode.

TABLE 3 Depth of Maximum Maximum Resistivity of Tissue Tissue the BodyPower Time Temperature Temperature (k-ohm·cm) (Watts) (Sec) (° C.) (cm)1.2 58 120 96.9 1.1 1.2 58 240 97.9 1.4 12 40 120 94.4 0.8 12 40 24095.0 1.0

The electrode body with higher resistivity body was observed to generatemore uniform temperature profiles, compared to a electrode body havingthe lower resistivity value. Due to additional heating generated at thetissue-electrode body interface with increased electrode bodyresistivity, less power was required to reach the same maximaltemperature. The consequence was that the lesion depth decreased.

Therefore, by specifying resistivity of the body 22, the physician cansignificantly influence lesion geometry. The use of a low-resistivitybody 22 results in deeper lesions, and vice versa. The following Table4, based upon empirical data, demonstrates the relationship between bodyresistivity and lesion depths.

TABLE 4 Resistivity Power Temperature Lesion Depth (ohm·cm) (Watts) (°C.) (cm) Time (sec) 850 94 97 1.2 120 1200 58 97 1.1 120 12,000 40 950.8 120

E. Shell Patterns

When it is expected that ablation will occur with the distal region ofbody 22 oriented in end-on contact with tissue, the shell 24 should, ofcourse, be oriented about the distal tip of the expandable-collapsiblebody 22. For this end-on orientation, the shell 24 may comprise acontinuous cap deposited upon the distal ⅓rd to ½ of the body 22, asFIG. 17 shows. However, when distal contact with tissue is contemplated,the preferred embodiment (see FIG. 20) segments the electricallyconductive shell 24 into separate energy transmission zones 122 arrangedin a concentric “bull's-eye” pattern about the distal tip of the body22.

The concentric bull's-eye zones 122 are formed by masking axially spacedbands on the distal region of the body 22, to thereby segment thedeposit of the electrically conductive shell 24 into the concentriczones 122. Alternatively, preformed foil shells 82 can be applied inaxially spaced bands on the distal region to form the segmented energytransmitting zones 122.

When it is expected that ablation will occur with the side region of thebody 22 oriented in contact with tissue, the shell 24 is preferablysegmented into axially elongated energy transmission zones 122, whichare circumferentially spaced about the distal ⅓rd to ½ of the body 22(see FIGS. 21 and 22).

The circumferentially spaced zones 122 are formed by maskingcircumferentially spaced areas of the distal region of the body 22, tothereby segment the deposit of the electrically conductive shell 24 intothe zones 122. Alternatively, preformed foil shells 82 can be applied incircumferentially spaced-apart relationship on the distal region to formthe segmented energy transmitting zones 122. Still alternatively, thecircumferentially segmented energy transmission zones 122 may take theform of semi-rigid pads carried by the expandable-collapsible body 22.Adjacent pads overlap each other when the body 22 is in its collapsedgeometry. As the body 22 assumes its expanded geometry, the pads spreadapart in a circumferential pattern on the body 22.

Preferably, regardless of the orientation of the zones 122 (bull's-eyeor circumferential), each energy transmission zone 122 is coupled to adedicated signal wire 26 or a dedicated set of signal wires 26. Thiswill be described later in greater detail. In this arrangement, thecontroller 32 can direct ablation energy differently to each zone 122according to prescribed criteria, as will also be described in greaterdetail later.

The above describes the placement of a shell 24 on the exterior of thebody 22. It should be appreciated that electrically conductive materialcan be deposited or otherwise affixed to the interior of the body 22.For example (as FIG. 44 shows), the interior surface of the body 22 cancarry electrodes 402 suitable for unipolar or bipolar sensing or pacing.Different electrode placements can be used for unipolar or bipolarsensing or pacing. For example, pairs of 2-mm length and 1-mm widthelectrodes 402 can be deposited on the interior surface of the body 22.Connection wires 404 can be attached to these electrodes 100. Preferablythe interelectrode distance is about 1 mm to insure good quality bipolarelectrograms. Preferred placements of these interior electrodes are atthe distal tip and center of the body 22. Also, when multiple zones areused, it is desired to have the electrodes 402 placed in between theablation regions.

It is also preferred to deposit opaque markers 406 on the interiorsurface of the body 22 so that the physician can guide the device underfluoroscopy to the targeted site. Any high-atomic weight material issuitable for this purpose. For example, platinum or platinum-iridium.can be used to build the markers 406. Preferred placements of thesemarkers 406 are at the distal tip and center of the structure 22.

F. Folding Segmented Shells

As FIGS. 21 and 22 show, segmented energy transmitting zones 122 arewell suited for use in association with folding expandable-collapsiblebodies 22, as previously described in connection with FIGS. 11A/B/C. Inthis arrangement, the regions that are masked before deposition of theelectrical conductive shell comprise the folding regions 52. In thisway, the regions 52 of the expandable-collapsible body 22 that aresubject to folding and collapse are those that do not carry anelectrically conductive shell 24. The electrically conductive shell 24is thereby protected against folding and stretching forces, which wouldcause creasing and current interruptions, or increases in resistance,thereby affecting local current densities and temperature conditions.

The selective deposition of the shell 24 in segmented patterns canitself establish predefined fold lines 52 on the body 22, withoutspecial molding of preformed regions of the body 22 (as FIGS. 11A/B/Ccontemplate). As FIGS. 23, 24A and 24B show, by controlling theparameters by which the shell segments 122 are deposited, predefinedfold lines 52 can be created at the borders between the shell segments122. These fold lines 52 are created due to the difference in thicknessbetween adjacent regions which are coated with the shell 24 and thosewhich are not.

More particularly, as FIGS. 23, 24A and 24B show, the region betweensegmented shell coatings will establish a fold line 52, when thedistance between the coatings (designated x in FIGS. 24A and B) isgreater than or equal to twice the thickness of the adjacent shellcoatings 122 (designated t in FIGS. 24A and B) divided by the tangent ofone half the minimum selected fold angle (designated α_(MIN) in FIG.24A). This fold line relationship is mathematically expressed asfollows: $x \succcurlyeq \frac{2t}{{Tan}\quad \alpha_{MIN}}$

The minimum selected fold angle 2α_(MIN) can vary according to theprofile of the body 22 desired when in the collapsed geometry.Preferably, the minimum fold angle 2α_(MIN) is in the range of 1° to 5°.

In this arrangement (see FIG. 23), the fold lines 52 created bycontrolled deposition of shell segments lie uniformly along (i.e.,parallel to) the long axis of the body 22 (designated 170 in FIG. 23).

The uncoated fold lines 52 created at the borders between the thickercoated shell segments 122 can also be characterized in terms of relativeelectrical resistivity values. The coated segments 122 of electricallyconductive material possess higher electrical conductivity than theuncoated fold lines 52. The resistivity of the fold lines 52, whichvaries inversely with conductivity, is thereby higher than theresistivity of the segments 122. To achieve the desired folding effectdue to differential coating, the region in which folding occurs shouldhave a resistivity that is greater than about ten times the resistivityof the segments 122 carrying electrically conductive material.

IV. Electrical Connection to Shell

It is necessary to electrically connect the shell 24 (or other ablationenergy transmitting material 84) to the radio frequency energy generator30 using the one or more signal wires 26. As before described, thesesignal wires 26, electrically connected to the shell 24, extend betweenthe body 22 and the external connectors 28 through the catheter tube 12.

The connection between the signal wires 26 and the shell 24, whetherdeposited, foil layered, or coextruded, must remain intact and free ofopen circuits as the expandable-collapsible body 22 and shell 24 changegeometries.

The electrical connection is preferably oriented proximate to thegeometric center of the pattern that the associated ablation zone 122defines. As FIGS. 25 to 27 show, the geometric center (designated GC)varies depending upon whether the zone 122 comprises a cap pattern (asFIG. 25 shows), or a circumferential segment pattern (as FIG. 26 shows),or a circumferential band or bull's-eye pattern (as FIG. 27 shows). Atleast one electrical connection should be present proximate to therespective geometric center of the pattern. This ensures that maximumcurrent density is distributed about the geometric center of the zoneand that similar current densities are distributed at the edges of thepattern.

Regardless of the shape of the pattern, additional electricalconnections are preferably made in each ablation zone. In the case of acap pattern or a segment pattern, the additional electrical connections(designated AC in, respectively, FIGS. 25 and 26) are distributeduniformly about the geometric center. In the case of a circumferentialband of a bull's-eye pattern, the additional electrical connections(designated ACG in FIG. 27) are distributed uniformly along the arcalong which the geometric center of the band lies.

Multiple electrical connections, at least one of which occurs proximateto the geometric center, provide more uniform current densitydistribution in the zone. These multiple connections are especiallyneeded when the resistivity of the shell 24 or of the correspondingpatterns is high. These connections prevent inefficient RF energydelivery due to RF voltage drops along parts of the shell 24 or thecorresponding patterns.

In a preferred embodiment of the cap or bull's-eye pattern (see FIGS.28A and 28B), multiple signal wires 26 are lead through the interior ofthe body 22 and out through a center aperture 74 in the distal fixture66. Multiple signal wires 26 are preferred, as multiple electricalconnections provide a more uniform current density distribution on theshell 24 than a single connection.

The signal wires 26 are enclosed within electrical insulation 160 (seeFIG. 28B) except for their distal ends. There, the electrical insulation160 is removed to expose the electrical conductor 162. The exposedelectrical conductor 162 is also preferably flattened by mechanicalmeans to provide an increased surface area. The flattened conductors 162are affixed by an electrically conductive adhesive proximate to thegeometric center and elsewhere at additional uniformly spaced intervalsabout it on the cap pattern, as well as along the geometric center ofthe concentric bands of the bull's-eye pattern, which the shell 24, whendeposited, will create.

It is preferred that the adhesive connections of the conductors 162 tothe body 22 be positioned, when possible, relatively close to anestablished support area on body 22, such as provided by the distalfixture 66. The support that the fixture 66 provides is a more secureattachment area for the electrical connections.

After the electrical connections are made, the shell 24 is deposited inthe desired pattern on the body 22, over the adhesively attachedconductors 162, in a manner previously described. The center aperture 74in the distal fixture 66 is sealed closed by adhesive or equivalentmaterial.

In an alternative embodiment (as FIGS. 29A/B show), the distal fixture66 can also be used to create a mechanical connection to electricallycouple a single signal wire 26 to the geometric center of the cap of thebull's-eye pattern. In the arrangement, the fixture 66 is made from anelectrically conductive material. As FIGS. 29A/B show, the signal wire26 is connected by spot welding, soldering, or electrically conductiveadhesive to the fixture 66 within the expandable-collapsible body 22. Anut 74 engaging a threaded fixture end 164 sandwiches the distal tip ofthe body 22 between it and the collar 68 (see FIG. 29B). Epoxy, whichcould be electrically conductive, could be used to further strengthenthe mechanical connection between the nut 74 and the body 22 sandwichedbeneath it. The shell 24 is next deposited on the body 22 and nut 74 ina manner previously described.

Alternatively, the shell 24 can be deposited on the body 22 beforeattachment of the nut 74. In this arrangement, the nut 74 sandwiches theshell 24 between it and the collar 68, mechanically establishing thedesired electrical connection between the signal wire 26 and the shell24.

Alternatively, instead of a threaded nut connection, a heat shrunk slipring of nickel titanium material can be used. Essentially, any riveting,swagging, electrically conductive plating, or bonding technique can beused to hold the shell 24 in contact against the collar 68.

It should be appreciated that additional solid fixtures 66 andassociated electrical connection techniques can be used in other regionsof the shell 22 distant from the distal tip of the body 22 to establishelectrical contact in the circumferential bands of the bull's-eyepattern or proximate the geometric center and elsewhere on thecircumferential segments. However, electrical connections can be made inthese regions without using fixtures 66 or equivalent structuralelements.

For example, as FIG. 30 shows, insulated signal wires 26 passed into theinterior of the body can be snaked through the body 22 at the desiredpoint of electrical connection. As before described, the electricalinsulation 160 of the distal end of the snaked-through wire 26 isremoved to expose the electrical conductor 162, which is also preferablyflattened. As also before described, the flattened conductors 162 areaffixed by an electrically conductive adhesive 172 to body 22, overwhich the shell 24 is deposited. Adhesive 172 is also preferably appliedin the region of the body 22 where the wire 26 passes to seal it. AsFIG. 31 shows, the same signal wire 26 can be snaked through the body 22multiple times to establish multiple electrical connections within thesame ablation zone.

In conjunction with any ablation zone pattern (see FIG. 32), theexpandable-collapsible body 22 can be formed as a laminate structure 90.The laminate structure 90 comprises a base layer 92, formed from anelectrically insulating material which peripherally surrounds theinterior of the body 22. The laminate structure 90 further includes oneor more intermediate layers 94 formed on the base layer 92. An ablationenergy wire 26 passes through each intermediate layer 94. Eachintermediate layer 94 is itself bounded by a layer 96 of electricallyinsulating material, so that the wires 26 are electrically insulatedfrom each other. The laminate structure 90 also includes an outer layer98 which is likewise formed from an electrically insulating material.

The laminate structure 90 can be formed by successively dipping a moldhaving the desired geometry in a substrate solution of electricallyinsulating material. The ablation energy wires 26 are placed onsubstrate layers between successive dippings, held in place byelectrically conductive adhesive or the like.

After molding the laminated structure 90 into the desired geometry, oneor more windows 100 are opened through the outer insulation layer 98 inthe region which the electrically conductive shell 24 will occupy. Eachwindow 100 exposes an ablation energy signal wire 26 in a chosen layer.

Various windowing techniques can be employed for this purpose. Forexample, CO₂ laser, Eximer laser, YAG laser, high power YAG laser, orother heating techniques can be used to remove insulation to the desiredlayer and thereby expose the desired signal wire 26.

After windowing, the formed expandable-collapsible body 22 is masked, asbefore described. The shell 24 of electrically conductive material isdeposited over the unmasked area, including the windows 100, which havebeen previously opened.

As FIG. 32 shows, the deposited shell 24 enters the windows 100, makingelectrically conductive contact with the exposed wires 26. A plating orother deposition process may be used in the window 100, beforedepositing the electrically conductive shell 24. The plating fills inthe window 100 to assure good electrical contact with the over-depositof shell 24.

FIG. 3D shows an alternative equivalent laminated structure, in whichthe chamber 124 occupies the interior of the body 22. This creates amultiple layer structure equivalent to the laminated structure justdescribed. An open intermediate layer 126 exists between the interior ofthe body 22 and the exterior of the chamber 124, through which signalwires 26 can be passed for electrical connection to the shell 24. Theelectrical connection can be made using either a distal fixture 66 or bysnaking the wires through the exterior body 22 (as FIG. 3D shows), bothof which have already been described.

V. Temperature Sensing

A. Connection of Temperature Sensors

As before described (see FIG. 1), a controller 32 preferably governs theconveyance of radio frequency ablation energy from the generator 30 tothe shell 24. In the preferred embodiment, the collapsible electrodestructure 20 carries one or more temperature sensing elements 104, whichare coupled to the controller 32. Temperatures sensed by the temperaturesensing elements 104 are processed by the controller 32. Based upontemperature input, the controller adjusts the time and power level ofradio frequency energy transmissions by the shell 24, to achieve thedesired lesion patterns and other ablation objectives.

The temperature sensing elements 104 can take the form of thermistors,thermocouples, or the equivalent. A temperature sensing element 104 maybe located within the distal fixture 66 to sense temperature at thedistal tip, as FIG. 33 shows. Alternatively, multiple temperaturesensing elements may be scattered at spaced apart locations on the shell24 or expandable-collapsible body 22, as FIG. 34 shows.

The connection of temperature sensing elements 104 to the shell 24 orexpandable-collapsible body 22 can be achieved in various ways.

As shown in FIG. 34, when the expandable-collapsible body 22 comprises athermally conductive material, the temperature sensing element(designated 104A in FIG. 34) can be attached to the interior surface ofthe body 22 in the region where measurement of exterior surfacetemperature is desired. A thermally conductive, but electricallyinsulating adhesive 106, can be used to secure the temperature sensingelement 104A to the inside of the body 22. The temperature sensingelement wires 110 extend through the catheter tube 12 for coupling(using a suitable connector 28, shown in FIG. 1)to the controller 32.

Alternatively, the temperature sensing element (designated 104B and 104Cin FIG. 34) can be attached to the exterior surface of the body 22 inthe region where measurement of temperatures is desired. As justdescribed, a thermally conductive, but electrically insulating adhesive106, can be used to secure the temperature sensing element to theoutside of the body 22.

As shown with element 104B, the electrically conductive shell 24 can bedeposited over the temperature sensing element 104B, in the mannerpreviously described. In this way, the temperature sensing element 104Bresides under the electrically conductive shell 24, and nodiscontinuities in the shell 24 are present.

Alternatively, as shown with element 104C, the element 104C can bemasked at the time the electrically conductive shell 24 is deposited. Inthis arrangement, there is no electrically conductive material over thetemperature sensing element 104C.

The signal wires 110 attached to the temperature sensing element 104Ccan be attached by electrically insulating adhesive to the outside ofthe expandable-collapsible body 22. Alternatively, as shown by element104B, the signal wires 110 can be brought from the interior of theexpandable-collapsible body 22 through the expandable-collapsible body22 for attachment by a thermally conductive, but electrically insulatingadhesive 106 to the outside of the body 22. The same type of adhesive106 can also be used to anchor in signal wires 110 to the inside of theexpandable-collapsible body 22.

As shown in FIG. 35, temperature sensing thermocouples 112 may also beintegrally formed by deposition on the expandable-collapsible body. Inthis embodiment, the body 22 comprises a laminated structure 114, likethat previously shown in FIG. 31, comprising a base layer 92, an outerlayer 98, and one or more intermediate layers 94. In the laminatestructure 114, the intermediate layers 94 formed in this structurethermocouple wires 116 (t-type or other combinations). Before depositingthe electrically conductive shell 24, windowing of the laminatedexpandable-collapsible body 116 in the manner previously describedexposes the thermocouple wires. A conducting material 118, which, for at-type thermocouple is copper or constantan, is deposited over theexposed thermocouple wires, forming the thermocouple 112. Anelectrically insulating material 120, like aluminum oxide or silicondioxide, is then applied over the thermocouple 112.

The electrically conducting shell 24 can be deposited over the formedthermocouple 112. In this way, the thermocouples reside under theelectrically conductive shell 24, and no discontinuities in the shell 24are present. Alternatively, as thermocouple 112A shows in FIG. 35, thethermocouple 112A can be masked at the time the electrically conductiveshell 24 is deposited. In this arrangement, there is no electricallyconductive material over the thermocouple 112A.

B. Location of Temperature Sensing Elements

Preferably, as FIGS. 20 and 21A/B show, multiple temperature sensingelements 104 are located on and about the shell 24 to ascertaintemperature conditions during radio frequency energy ablation. Thecontroller 32 uses temperature information from temperature sensingelements to control the transmission of ablation energy by the shell 24.

Generally speaking, at least one temperature sensing element 104 ispreferably placed proximal to the geometric center of the energytransmitting shell 24. When the shell 24 is segmented (as FIGS. 20 and21A/B show), at least one temperature sensing element 104 should beproximal to the geometric center of each energy transmitting segment122.

Preferably, as FIGS. 20 and 21A/B further show, temperature sensingelements 104 are also placed along the edges of the shell 24, where itadjoins a masked, electrically non-conductive region of the body 22.When the shell 24 is segmented, temperature sensing elements 104 shouldbe placed along the edge of each energy transmitting segment 122. Highcurrent densities occur along these regions where energy transmittingmaterial adjoins non-energy transmitting material. These edge effectslead to higher temperatures at the edges than elsewhere on the shell 24.Placing temperature sensing elements 104 along the edges assures thatthe hottest temperature conditions are sensed.

In the case of a shell 24 segmented into adjacent energy transmittingzones 122, it is also desirable to place at least one temperaturesensing element 104 between adjacent energy transmitting zones, as FIGS.20 and 21A/B show. Placing multiple temperature sensing elements 104 inthe segments 122, between the segments 122, and along the edges of thesegments 122 allows the controller 32 to best govern power distributionto the multiple segments 122 based upon predictions of hottesttemperature conditions. Further details of the use of multipletemperature sensing elements, including edge temperature sensingelements, and the use of temperature prediction methodologies, are foundin copending U.S. patent application Ser. No. 08/439,824, filed May 12,1995, and entitled “Systems and Methods for Controlling Tissue AblationUsing Multiple Temperature Sensing Elements.”

The presence of segmented energy transmission zones 122, each with itsown prescribed placement of temperature sensing elements 104, allows thecontroller 32 to govern the delivery of power to each zone 122separately. The controller 32 is thereby able to take into account andreact to differences in convective cooling effects in each zone 122 dueto blood flow, differences in contact pressure and surface area betweeneach zone 122 and the tissue that it contacts, and other nonlinearfactors affecting the power required to heat tissue adjacent each zone122 to a predetermined temperature to achieve the desired lesiongeometry and pattern.

Thus, whereas for any given transmission zone (like the continuous,non-segmented shell 24 shown in FIG. 25 or each segmented zone 122 shownin FIGS. 26 and 27), it is desirable to allow some contact with theblood pool to allow beneficial convective cooling effects, it is notdesirable that any given zone contact only or substantially only theblood pool. Loss of power into the blood pool with no tissue ablationeffects occurs. With segmented zones 122, it is possible to sense, usingthe temperature sensing elements 104, where insubstantial tissue contactexists. It is thereby possible to sense and to channel available poweronly to those zones 122 where substantial tissue contact exists. Furtherdetails of tissue ablation using segmented electrode structures aredisclosed in copending U.S. patent application Ser. No. 08/139,304,filed Oct. 19, 1993 and entitled “Systems and Methods for CreatingLesions in Body Tissue Using Segmented Electrode Assemblies.”

Further details of the use of multiple ablation energy transmitterscontrolled using multiple temperature sensing elements are disclosed incopending U.S. patent application Ser. No. 08/286,930, filed Aug. 8,1994, and entitled “Systems and Methods for Controlling Tissue AblationUsing Multiple Temperature Sensing Elements”.

FIG. 36 shows a preferred representative embodiment when the shell 24comprises a continuous cap pattern. In this arrangement, the structure20 carries five temperature sensing elements 104 spaced apart on theshell 24. The temperature sensing elements 104 are connected in aselected one or more of the manners previously described.

Preferably, sensing elements Tn₁ and Tn₂ are placed at diametricallyopposite regions at the most proximal edge of the shell 24. Sensingelements Tm₁ and Tm₂ are placed at diametrical sides of the middleregion of the shell 24, for example, at about 50% of the radius of thestructure. The sensor Tc is placed proximal the geometric center of theshell 24. All temperature sensors are coupled to a temperaturecontroller, which processes information from the sensors.

In this arrangement, the temperature controller 32 infers the percentageof tissue contact with the shell 24 contact based upon where significantincreases in temperature conditions from an established baseline level(for example, 37° C.) are sensed on the shell 24. These increasedtemperature conditions indicate the absence of convective coolingeffects, as would occur with contact with the blood pool, therebysuggesting tissue contact. As the preceding Tables 1 and 2 show,percentage of contact between the shell 24 and tissue dictate effectivepower levels to achieve the type of lesion desired.

The relationship between percentage shell-tissue contact and powerdesired for a given lesion characteristic can be based upon empirical ortheoretical data in the manner set forth in the preceding Example. Theserelationships can be set forth in look up table format or incorporatedin equivalent decision matrices, which the controller 32 retains inmemory.

For example, if large deep lesions are desired, significant increase intemperature above the baseline at Tc, but not elsewhere, indicates a 20%tissue contact condition, and a first power level is commanded for thegenerator 30 based upon the selected power criteria. Significantincrease in temperature above the baseline also at Tm₁ and Tm₂ indicatesa 50% tissue contact condition, and second power level less than thefirst is commanded for the generator 30 based upon the selected powercriteria. Significant increase in temperature above the baseline also atTn₁ and Tn₂ indicates a 100% tissue contact condition, and third powerlevel less than the second is commanded based upon the selected powercriteria.

FIG. 37 shows a preferred representative embodiment when the shell 24comprises a circumferentially spaced, segmented pattern. In thisarrangement, the structure 20 carries at least four temperature sensingelements on each shell segment.

The sensor Tc is common to all segments and is located at the distal endof the pattern. The sensor T_(GC) is located at the geometric center ofeach segment, while the sensors T_(E1) and T_(E2) are located alongopposite edges of each segment, where the shell 24 adjoins thenon-electrically conductive regions separating the segments. Anadditional sensor T_(M) is preferably also located generally between thesegments for the reasons discussed before.

FIG. 38 shows a preferred representative embodiment when the shell 24comprises a bull's-eye pattern. Sensors T_(GC) are located at thegeometric center of each segment of the pattern, while the sensorsT_(E1) and T_(E2) are located along opposite edges of each segment,where the shell 24 adjoins the non-electrically conductive regionsseparating the segments. An additional sensor T_(M) is preferably alsolocated generally between the segments for the reasons discussed before.

VI. Active Cooling

The capability of the shell 24 to form large lesions can be enhanced byactively cooling the shell 24 while transmitting ablation energy.

Active cooling can be accomplished by the use of multiple lumens tocycle a cooled fluid through the expandable-collapsible body 22 whiletransmitting ablation energy. Alternatively, a high pressure gas can betransported by the lumens for expansion within theexpandable-collapsible body to achieve a comparable active coolingeffect. In yet another alternative arrangement, the cooled medium can beconveyed outside the expandable-collapsible body 22 to achieve an activecooling effect.

With active cooling, more power can be applied, while maintaining thesame maximum tissue temperature conditions, thereby creating larger anddeeper lesions. With active cooling, the percentage contact of the shell24 with tissue relative to blood can be increased above 50%, and may beas much as 100%.

Further details concerning the use of active cooling to enhance lesionformation are found in copending U.S. patent application Ser. No.08/431,790, filed May 1, 1995, and entitled “Systems and Methods forobtaining Desired Lesion Characteristics While Ablating Body Tissue”.

It should be appreciated that the entire surface of the shell 24 neednot be cooled to achieve at least some of the benefits of activecooling. For example, only selected regions of the shell 24 which areprone to localized edge heating effects, as previously described, can besubjected to active cooling. The edge effects on current densities occurat the boundary between the shell 24 and expandable-collapsible body 22that is free of the shell 24 create higher temperatures. Localizedcooling of these edge regions can help minimize the effects of hot spotson lesion formation.

In this arrangement, as FIG. 39 shows, a pattern of small holes 174 iscreated in the region between segmented shell patterns 122. Liquidcooling medium is perfused from inside the body 22 through the holes 174to provide localized cooling adjacent the edges of the shell segments122. It should be appreciated that hole patterns 174 could be usedelsewhere on the body 22 to provide active cooling effects.

As FIGS. 39, 40A and 40B show, the selective establishment of holepatterns 174 on the body 22 can also itself establish predefined foldlines 52, eliminating the need to specially mold preformed foldingregions the body 22. The pattern of small holes 174 create fold lines 52by the removal of material, thereby increasing the flexibility of thebody 22 along the holes 174 between adjacent regions 122. In thisarrangement (see FIG. 39), the fold lines 52 created by hole patterns174 lie uniformly along (i.e., parallel to) the long axis of the body22.

VII. Obtaining Desired Lesion Characteristics

As Tables 1 and 2 in the foregoing Example demonstrate, the sameexpandable-collapsible electrode structure 20 is able to selectivelyform lesions that are either wide and shallow or large and deep. Variousmethodologies can be used to control the application of radio frequencyenergy to the shell 24 of the body 20 to achieve this result.

A. D_(50C) Function

In one representative embodiment, the controller 32 includes an input300 for receiving from the physician a desired therapeutic result interms of (i) the extent to which the desired lesion should extendbeneath the tissue-electrode interface to a boundary depth betweenviable and nonviable tissue and/or (ii) a maximum tissue temperaturedeveloped within the lesion between the tissue-electrode interface andthe boundary depth.

The controller 32 also includes a processing element 302, which retainsa function that correlates an observed relationship among lesionboundary depth, ablation power level, ablation time, actual or predictedsub-surface tissue temperature, and electrode temperature. Theprocessing element 302 compares the desired therapeutic result to thefunction and selects an operating condition based upon the comparison toachieve the desired therapeutic result without exceeding a prescribedactual or predicted sub-surface tissue temperature.

The operating condition selected by the processing element 302 cancontrol various aspects of the ablation procedure, such as controllingthe ablation power level, the rate at which the structure 20 is activelycooled, limiting the ablation time to a selected targeted ablation time,limiting the ablation power level subject to a prescribed maximumablation power level, and/or the orientation of the shell 24, includingprescribing a desired percentage contact between the shell 24 andtissue. The processing element 302 can rely upon temperature sensorscarried by or otherwise associated with the expandable-collapsiblestructure 20 that penetrate the tissue to sense actual maximum tissuetemperature. Alternatively, the processing element 302 can predictmaximum tissue temperature based upon operating conditions.

In the preferred embodiment, the electrode structure 20 carries at leastone temperature sensing element 104 to sense instantaneous localizedtemperatures (T1) of the thermal mass of the shell 24. The temperatureT1 at any given time is a function of the power supplied to the shell 24by the generator 30 and the rate at which the shell 24 is cooled, eitherby convective cooling by the blood pool, or active cooling by anothercooling medium brought into contact with the shell 24, or both.

The characteristic of a lesion can be expressed in terms of the depthbelow the tissue surface of the 50° C. isothermal region, which will becalled D_(50C). The depth D_(50C) is a function of the physicalcharacteristics of the shell 24 (that is, its electrical and thermalconductivities and size); the percentage of contact between the tissueand the shell 24; the localized temperature T1 of the thermal mass ofthe shell 24; the magnitude of RF power (P) transmitted by the shell 24into the tissue, and the time (t) the tissue is exposed to the RF power.

For a desired lesion depth D50C, additional considerations of safetyconstrain the selection of an optimal operating condition among theoperating conditions listed in the matrix. The principal safetyconstraints are the maximum tissue temperature TMAX and maximum powerlevel PMAX.

The maximum temperature condition TMAX lies within a range oftemperatures which are high enough to provide deep and wide lesions(typically between about 85° C. and 95° C.), but which are safely belowabout 100° C., at which tissue desiccation or tissue micro-explosionsare known to occur. It is recognized that TMAX will occur a distancebelow the electrode-tissue interface between the interface and D_(50C).

The maximum power level PMAX takes into account the physicalcharacteristics of the electrode and the power generation capacity ofthe RF generator 30.

These relationships can be observed empirically and/or by computermodeling under controlled real and simulated conditions, as theforegoing examples illustrate. The D50C function for a given shell 24can be expressed in terms of a matrix listing all or some of theforegoing values and their relationship derived from empirical dataand/or computer modeling.

The processing element 302 includes in memory this matrix of operatingconditions defining the D50C temperature boundary function, as describedabove for t=120 seconds and TMAX=95° C. and for an array of otheroperating conditions.

The physician also uses the input 300 to identify the characteristics ofthe structure 20, using a prescribed identification code; set a desiredmaximum RF power level PMAX; a desired time t; and a desired maximumtissue temperature TMAX.

Based upon these inputs, the processing element 302 compares the desiredtherapeutic result to the function defined in the matrix (as exemplifiedby the above Tables 1 and 2). The master controller 58 selects anoperating condition to achieve the desired therapeutic result withoutexceeding the prescribed TMAX by controlling the function variables.

This arrangement thereby permits the physician, in effect, to“dial-a-lesion” by specifying a desired D_(50C). By taking into accountthe effects of convective cooling based upon percentage of shell contactwith tissue and by using active cooling in association with time andpower control, the processing element can achieve the desired D_(50C)without the need to sense actual tissue temperature conditions.

Further details of deriving the D_(50C) function and its use inobtaining a desired lesion pattern are found in copending U.S.application Ser. No. 08/431,790, filed May 1, 1995, entitled “Systemsand Methods for Obtaining Desired Lesion Characteristics While AblatingBody Tissue,” which is incorporated herein by reference.

B. Predicting Maximum Tissue Temperature

The structure 20 is cooled either by convective blood flow (dependingupon percentage contact between the shell 24 and tissue), or by activelyusing another cooling medium, or both. The level of RF power deliveredto the cooled structure 20 and/or the cooling rate can be adjusted basedupon a prediction of instantaneous maximum tissue temperature, which isdesignated Ψ_(MAX) (t).

In a preferred implementation, the prediction of Ψ_(MAX) is derived by aneural network, which has as inputs a prescribed number of previouspower levels, previous rates at which heat has been removed to cool thestructure 20, and previous shell temperature.

The heat removal rate is identified by the expression Å, where

Å=c×ΔT×RATE

where:

c is the heat capacity of the cooling medium used (in Joules (J) perkilogram (kg) Kelvin (K), or J/kg K)

ΔT is the temperature drop in the cooling medium during passing throughthe structure 20 (K), and

RATE is the mass flow rate of the cooling medium through the structure(kg/sec).

The heat generated by the structure 20 into the tissue is the differencebetween the heat generated by Joule effect and the heat removed bycooling. At a given localized shell temperature T1 and flow rate ofcooling medium, the magnitude of Å increases as RF power delivered tothe shell 24 increases. Together, T1 and Å represent an indirectmeasurement of how rapidly the sub-surface tissue temperature ischanging. Together, T1 and Å are therefore predictive of the depth andmagnitude of the hottest sub-surface tissue temperature Ψ_(MAX), andthus indirectly predictive of the lesion boundary depth D_(50C). Largedeep lesions are predicted when T1 is maintained at a low relativetemperature (by controlling cooling rate) and the maximal predictedtissue temperature, TMAX, is maintained at approximately 85° C. to 95°C. by controlling RF power. Likewise, more shallow lesions are predictedwhen T1 is maintained at a high relative temperature and TMAX ismaintained at lower values.

Further details of deriving the this prediction function and its use inobtaining a desired lesion pattern are found in copending U.S.application Ser. No. 08/431,790, filed May 1, 1995, entitled “Systemsand Methods for Obtaining Desired Lesion Characteristics While AblatingBody Tissue,” which is incorporated herein by reference.

C. Segmented Shells: Duty Cycle Control

Various RF energy control schemes can also be used in conjunction withsegmented shell patterns shown in FIG. 20 (the axially spaced,bull's-eye pattern of zones) and FIGS. 21 and 22 (the circumferentiallyspaced zones). For the purpose of discussion, the zones (which will alsobe called electrode regions) 122 will be symbolically designated E(J),where J represents a given zone 122 (J=1 to N).

As before described, each electrode region E(J) has at least onetemperature sensing element 104, which will be designated S(J,K), whereJ represents the zone and K represents the number of temperature sensingelements on each zone (K=1 to M).

In this mode, the generator 30 is conditioned through an appropriatedpower switch interface to deliver RF power in multiple pulses of dutycycle 1/N.

With pulsed power delivery, the amount of power (P_(E(J))) conveyed toeach individual electrode region E(J) is expressed as follows:

P_(E(J))αAMP_(E(J)) ²×DUTYCYCLE_(E(J))

where:

AMP_(E(J)) is the amplitude of the RF voltage conveyed to the electroderegion E(J), and

DUTYCYCLE_(E(J)) is the duty cycle of the pulse, expressed as follows:${DUTYCYCLE}_{E{(J)}} = \frac{{TON}_{E{(J)}}}{{TON}_{E{(J)}} + {TOFF}_{E{(J)}}}$

where:

TON_(E(J)) is the time that the electrode region E(J) emits energyduring each pulse period,

TOFF_(E(J)) is the time that the electrode region E(J) does not emitenergy during each pulse period.

The expression TON_(E(J))+TOFF_(E(J)) represents the period of the pulsefor each electrode region E(J).

In this mode, the generator 30 can collectively establish duty cycle(DUTYCYCLE_(E(J))) of 1/N for each electrode region (N being equal tothe number of electrode regions).

The generator 30 may sequence successive power pulses to adjacentelectrode regions so that the end of the duty cycle for the precedingpulse overlaps slightly with the beginning of the duty cycle for thenext pulse. This overlap in pulse duty cycles assures that the generator30 applies power continuously, with no periods of interruption caused byopen circuits during pulse switching between successive electroderegions.

In this mode, the temperature controller 32 makes individual adjustmentsto the amplitude of the RF voltage for each electrode region(AMP_(E(J))), thereby individually changing the power P_(E(J)) ofablating energy conveyed during the duty cycle to each electrode region,as controlled by the generator 30.

In this mode, the generator 30 cycles in successive data acquisitionsample periods. During each sample period, the generator 30 selectsindividual sensors S(J,K), and temperature codes TEMP(J) (highest ofS(J,K)) sensed by the sensing elements 104, as outputted by thecontroller 32.

When there is more than one sensing element associated with a givenelectrode region (for example, when edge-located sensing elements areused, the controller 32 registers all sensed temperatures for the givenelectrode region and selects among these the highest sensed temperature,which constitutes TEMP(J).

In this mode, the generator 30 compares the temperature TEMP(J) locallysensed at each electrode E(J) during each data acquisition period to aset point temperature TEMP_(SET) established by the physician. Basedupon this comparison, the generator 30 varies the amplitude AMP_(E(J))of the RF voltage delivered to the electrode region E(J), whilemaintaining the DUTYCYCLE_(E(J)) for that electrode region and all otherelectrode regions, to establish and maintain TEMP(J) at the set pointtemperature TEMP_(SET).

The set point temperature TEMP_(SET) can vary according to the judgmentof the physician and empirical data. A representative set pointtemperature for cardiac ablation is believed to lie in the range of 40°C. to 95° C., with 70° C. being a representative preferred value.

The manner in which the generator 30 governs AMP_(E(J)) can incorporateproportional control methods, proportional integral derivative (PID)control methods, or fuzzy logic control methods.

For example, using proportional control methods, if the temperaturesensed by the first sensing element TEMP(1)>TEMP_(SET), the controlsignal generated by the generator 30 individually reduces the amplitudeAMP_(E(1)) of the RF voltage applied to the first electrode region E(1),while keeping the duty cycle DUTYCYCLE_(E(1)) for the first electroderegion E(1) the same. If the temperature sensed by the second sensingelement TEMP(2)<TEMP_(SET), the control signal of the generator 30increases the amplitude AMP_(E(2)) of the pulse applied to the secondelectrode region E(2), while keeping the duty cycle DUTYCYCLE_(E(2)) forthe second electrode region E(2) the same as DUTYCYCLE_(E(1)), and soon. If the temperature sensed by a given sensing element is at the setpoint temperature TEMP_(SET), no change in RF voltage amplitude is madefor the associated electrode region.

The generator continuously processes voltage difference inputs duringsuccessive data acquisition periods to individually adjust AMP_(E(J)) ateach electrode region E(J), while keeping the collective duty cycle thesame for all electrode regions E(J). In this way, the mode maintains adesired uniformity of temperature along the length of the ablatingelement.

Using a proportional integral differential (PID) control technique, thegenerator takes into account not only instantaneous changes that occurin a given sample period, but also changes that have occurred inprevious sample periods and the rate at which these changes are varyingover time. Thus, using a PID control technique, the generator willrespond differently to a given proportionally large instantaneousdifference between TEMP (J) and TEMP_(SET), depending upon whether thedifference is getting larger or smaller, compared to previousinstantaneous differences, and whether the rate at which the differenceis changing since previous sample periods is increasing or decreasing.

Further details of individual amplitude/collective duty cycle controlfor segmented electrode regions based upon temperature sensing are foundin copending U.S. application Ser. No. 08/439,824, filed May 12, 1995and entitled “Systems and Methods for Controlling Tissue Ablation UsingMultiple Temperature Sensing Elements,” which is incorporated herein byreference.

D. Segmented Shells: Differential Temperature Disabling

In this control mode, the controller 32 selects at the end of each dataacquisition phase the sensed temperature that is the greatest for thatphase (TEMP_(SMAX)). The controller 32 also selects for that phase thesensed temperature that is the lowest (TEMP_(SMIN)).

The generator compares the selected hottest sensed temperatureTEMP_(SMAX) to a selected high set point temperature TEMP_(HISET). Thecomparison generates a control signal that collectively adjusts theamplitude of the RF voltage for all electrodes using proportional, PID,or fuzzy logic control techniques.

In a proportion control implementation scheme:

(i) If TEMP_(SMAX)>TEMP_(HISET), the control signal collectivelydecreases the amplitude of the RF voltage delivered to all segments;

(ii) If TEMP_(SMAX)<TEMP_(HISET), the control signal collectivelyincreases the amplitude of the RF voltage delivered to all segments;

(iii) If TEMP_(SMAX)TEMP_(HISET), no change in the amplitude of the RFvoltage delivered to all segments.

It should be appreciated that the generator can select for amplitudecontrol purposes any one of the sensed temperatures TEMP_(SMAX),TEMP_(SMIN), or temperatures in between, and compare this temperaturecondition to a preselected temperature condition.

The generator governs the delivery of power to the segments based upondifference between a given local temperature TEMP (J) and TEMP_(SMIN).This implementation computes the difference between local sensedtemperature TEMP(J) and TEMP_(SMIN) and compares this difference to aselected set point temperature difference ΔTEMP_(SET). The comparisongenerates a control signal that governs the delivery of power to theelectrode regions.

If the local sensed temperature TEMP(J) for a given electrode regionE(J) exceeds the lowest sensed temperature TEMP_(SMIN) by as much as ormore than ΔTEMP_(SET) (that is, if TEMP(J)−TEMP_(SMIN)≧ΔTEMP_(SET)), thegenerator turns the given segment E(J) off. The generator turns thegiven segment E(J) back on when TEMP(J)−TEMP_(SMIN)<ΔTEMP_(SET).

Alternatively, instead of comparing TEMP(J) and TEMP_(SMIN), thegenerator can compare TEMP_(SMAX) and TEMP_(SMIN). When the differencebetween TEMP_(SMAX) and TEMP_(SMIN) equals or exceeds a predeterminedamount ΔTEMP_(SET), the generator turns all segments off, except thesegment where TEMP_(SMIN) exists. The controller 231 turns thesesegments back on when the temperature difference between TEMP_(SMAX) andTEMP_(SMIN) is less than ΔTEMP_(SET).

Further details of the use of differential temperature disabling arefound in copending U.S. patent application Ser. No. 08/286,930, filedAug. 8, 1994, and entitled “Systems and Methods for Controlling TissueAblation Using Multiple Temperature Sensing Elements,” which isincorporated herein by reference.

E. Segmented Shells (Predicted Hottest Temperature)

Because of the heat exchange between the tissue and the electrode region122, the temperature sensing elements 104 may not measure exactly themaximum temperature at the region 122. This is because the region ofhottest temperature occurs beneath the surface of the tissue at a depthof about 0.5 to 2.0 mm from where the energy emitting electrode region122 (and the associated sensing element 104) contacts the tissue. If thepower is applied to heat the tissue too quickly, the actual maximumtissue temperature in this subsurface region may exceed 100° C. and leadto tissue desiccation and/or micro-explosions.

FIG. 43 shows an implementation of a neural network predictor 400, whichreceives as input the temperatures sensed by multiple sensing elementsS(J,K) at each electrode region, where J represents a given electroderegion (J=1 to N) and K represents the number of temperature sensingelements on each electrode region (K=1 to M). The predictor 400 outputsa predicted temperature of the hottest tissue region T_(MAXPRED)(t). Thegenerator 30 derives the amplitude and duty cycle control signals basedupon T_(MAXPRED)(t), in the same manner already described using TEMP(J).

The predictor 400 uses a two-layer neural network, although more hiddenlayers could be used. As shown in FIG. 43, the predictor 300 includes afirst and second hidden layers and four neurons, designated N_((L,X)),where L identifies the layer 1 or 2 and X identifies a neuron on thatlayer. The first layer (L=1) has three neurons (X=1 to 3), as followsN_((1,1)); N_((1,2)); and N_((1,3)). The second layer (L=2) comprisesone output neuron (X=1), designated N_((2,1)).

Temperature readings from the multiple sensing elements, only two ofwhich—TS1(n) and TS2(n)—are shown for purposes of illustration, areweighed and inputted to each neuron N_((1,1)); N_((1,2)); and N_((1,3))of the first layer. FIG. 43 represents the weights as W^(L) _((k,N)),where L=1; k is the input sensor order; and N is the input neuron number1, 2, or 3 of the first layer.

The output neuron N_((2,1)) of the second layer receives as inputs theweighted outputs of the neurons N_((1,1))); N_((1,2)); and N_((1,3)).FIG. 43 represents the output weights as W^(L) _((O,X)), where L=2; O isthe output neuron 1, 2, or 3 of the first layer; and X is the inputneuron number of the second layer. Based upon these weighted inputs, theoutput neuron N_((2,1)) predicts T_(MAXPRED)(t).

Alternatively, a sequence of past reading samples from each sensor couldbe used as input. By doing this, a history term would contribute to theprediction of the hottest tissue temperature.

The predictor 400 must be trained on a known set of data containing thetemperature of the sensing elements TS1 and TS2 and the temperature ofthe hottest region, which have been previously acquired experimentally.For example, using a back-propagation model, the predictor 400 can betrained to predict the known hottest temperature of the data set withthe least mean square error. Once the training phase is completed, thepredictor 300 can be used to predict T_(MAXPRED)(t).

Other types of data processing techniques can be used to deriveT_(MAXPRED)(t). See, e.g., copending patent application Ser. No.08/266,934, filed Jun. 27, 1994, and entitled “Tissue Heating andAblation Systems and Methods Using Predicted Temperature for Monitoringand Control.”

The illustrated and preferred embodiments use digital processingcontrolled by a computer to analyze information and generate feedbacksignals. It should be appreciated that other logic control circuitsusing micro-switches, AND/OR gates, invertors, analog circuits, and thelike are equivalent to the micro-processor controlled techniques shownin the preferred embodiments.

VIII. Capacitive Coupling

In the preceding embodiments, the electrode structure 20 transmitsablation energy to tissue by exposing tissue to an electricallyconductive surface 24 carried about the exterior of theexpandable-collapsible body 22. The alternative embodiments shown inFIGS. 41A and 42A include an electrode structure 176 comprising anexpandable-collapsible body 178 having an exterior free of anelectrically conductive surface. In these embodiments, the body 178 iscapacitively coupled to tissue for the purpose of transmitting ablationenergy.

In the embodiment shown in FIG. 41A, the expandable-collapsible body 178is molded in the same fashion as the body 22 previously described. Thebody 178 includes an electrically conductive structure 180 in contactwith at least a portion of the interior surface 182 of the body 178.

The interior conductive structure 180 can be assembled in various ways.In the embodiment shown in FIG. 41A, the structure 180 comprises aninterior shell 184 of electrically conductive material deposited on atleast a portion of the interior surface 182 of the body 178. Like theexterior shell 24 previously described, the interior shell 184 comprisesa material having a relatively high electrical conductivity, as well asa relative high thermal conductivity, such as gold, platinum,platinum/iridium, among others. The shell 184 is preferably depositedupon the exterior of the body 178 after molding using deposition processlike sputtering, vapor deposition, ion beam deposition, electroplatingover a deposited seed layer, or a combination of these processes. Thebody 178 is then everted in the manner previously described (as FIG. 16Bshows) to place the deposited shell 184 inside the everted body 178. Oneor more signal wires 186 are coupled to the interior shell 184 usingelectrically conductive adhesive, soldering, or equivalent connectiontechniques.

The body 178 can be caused to assume expanded and collapsed geometriesby the introduction of an air or liquid inflation medium, as previouslydescribed. Alternatively, the body 178 can employ any previouslydescribed interior support structure 44 to affect expansion andcollapse. The support structures 44 could also be electricallyconductive to affect capacitive coupling, with or without the presenceof the deposited shell 184. For example, an electrically conductiveinterior resilient mesh structure (like that shown in FIG. 6), or askeleton of flexible, electrically conductive spline elements (like thatshown in FIG. 4), or an open cell foam structure coated with anelectrically conductive material (like that shown in FIG. 9), can beused both to provide interior support and to provide capacitive couplingbetween signal wires 186 and tissue, with or without the presence of thedeposited interior shell 184. In these alternative arrangements, one ormore signal wires 184 are coupled to the electrically conductive supportstructures.

FIG. 41B shows the electrical equivalent circuit 188 of the capacitivecoupling effect that the structure 176 in FIG. 41A provides. In theelectrical path 190 that the ablation energy 192 follows, the interface194 formed among the expandable-collapsible body 178, the conductivestructure 180 contacting the inside the body 178, and the tissue 196contacting the outside of the body 178 functions as a capacitor(designated C), whose impedance X_(C) is expressed as:$X_{C} = \frac{1}{2\pi \quad {fC}}$

where:

f is the frequency of the radio frequency ablation energy 192, and$C = {ɛ\frac{s}{t}}$

where

ε is the dielectric constant of the material of theexpandable-collapsible body 178, which ranges from about 1.2 to about10.0 (multiplied by 8.85×10⁻¹² Farads per meter) for most plasticmaterials,

s is the surface area of the electrically conductive structure 184, and

t is the thickness of the body 178 located between the electricallyconductive structure 180 and the contacted tissue 196.

In the electrical path 190 that the ablation energy 192 follows, thetissue 196 functions as a resistor (designated R_(TISSUE)) seriescoupled to C. Typically, R_(TISSUE) is about 100 ohms.

To have efficient capacitive coupling to the tissue, X_(C) of thestructure 180 must be less than R_(TISSUE). This relationship assuresthat the desired ohmic heating effect is concentrated in tissue.

To maximize the capacitive coupling effect, it is thereby important touse ablation energy at higher frequencies (for example, between 10 and20 Mhz). It is also important to aim to maximize C as much as possible,by controlling thickness of the body 178 as well as by maximizing asmuch as possible the surface area of contact with the electricallyconductive structure 180 inside the body 178. For this reason, acontinuous electrically conductive shell 182 or equivalent meshstructure are preferred, compared to a more open spline elementstructure. However, a more dense, conductive spline element structurehaving many spline elements and/or large surface area splines could beused to maximize C, if desired.

FIG. 42A shows an alternative embodiment of an expandable-collapsibleelectrode structure 198 that provides capacitive coupling to tissue. Thestructure 198 comprises an interior electrode 200 of electricallyconductive material located within the interior of the body 178. Theinterior electrode 200 comprises a material having a relatively highelectrical conductivity, as well as a relatively high thermalconductivity, such as gold, platinum, platinum/iridium, among others. Asignal wire 202 is coupled to the electrode to conduct ablation energyto it.

In this embodiment, a hypertonic (i.e., 9%) saline solution 204 fillsthe interior of the body 178. The saline solution 204 serves as anelectrically conductive path to convey radio frequency energy from theelectrode 200 to the body 178. The saline solution 204 also serves asthe inflation medium, to cause the body 178 to assume the expandedgeometry. Removal of the saline solution 204 causes the body 178 toassume the collapsed geometry.

FIG. 42B shows the electrically equivalent circuit 206 of the capacitivecoupling effect that the structure 198 shown in FIG. 42A provides. Inthe electrical path 208 that the ablation energy 210 follows, theinterface 212 formed among the expandable-collapsible body 178, thehypertonic saline solution 204 contacting the inside the body 178, andthe tissue 196 contacting the outside of the body 178 functions as acapacitor (designated C), whose impedance X_(C) is expressed as:$X_{C} = \frac{1}{2\pi \quad {fC}}$

where:

f is the frequency of the radio frequency ablation energy 210, and$C = {ɛ\frac{S_{B}}{t}}$

where

ε is the dielectric constant of the material of the body 178,

S_(B) is the area of the body 178 contacting the hypertonic salinesolution 204, and

t is the thickness of the body 178 located between the electricallyconductive saline solution 204 and the tissue 196.

In the electrical path that the ablation energy follows, the tissue 196functions as a resistor (designated R_(TISSUE)) series coupled to C,which value is about 100 ohms. The path 216 through the hypertonicsaline 204 between the interior electrode 200 and the interior surface214 of the body 178 also functions as a resistor (designated R_(PATH))series coupled to C. The value of R_(PATH) is expressed:$R_{PATH} = {\frac{K}{S_{E}}\rho}$

where:

K is a constant that depends upon the geometry of the structure 198,

S_(E) is the surface area of the interior electrode 200, and

ρ is the resistivity of the hypertonic saline 204.

The following relationship establishes efficient capacitive couplingbetween the structure 198 and tissue 196 to achieve the desired ohmictissue heating effect:

{square root over (R_(PATH) ²+L +X_(C) ²+L )}<R_(TISSUE)

The use of capacitive coupling provides structural benefits. It isolatespossible shell adherence problems to inside the body 178 of thestructure 176, where flaking and chipping of the shell 184 can beretained out of the blood pool. Capacitive coupling also avoidspotential problems that tissue sticking to exterior conductive materialscould create.

In addition to these structural benefits, the temperature control of theablation process (as described above in conjunction with the structure20) is improved using capacitive coupling. When using a metal surface toablate tissue, the tissue-electrode interface is convectively cooled bysurrounding blood flow. Due to these convective cooling effects, theregion of maximum tissue temperature is located deeper in the tissue. Asa result, the temperature conditions sensed by sensing elementsassociated with metal electrode elements do not directly reflect actualmaximum tissue temperature. In this situation, maximum tissuetemperature conditions must be inferred or predicted from actual sensedtemperatures, as set forth above. Using capacitive coupling instructures 176 or 198, convective cooling of the tissue-electrodeinterface by the surrounding blood flow is minimized. As a result, theregion of maximum temperature is located at the interface between tissueand the porous electrode. As a result, the temperature conditions sensedby sensing elements associated with the capacitively coupled structures176 or 198 will more closely reflect actual maximum tissue.

IX. Conductive Polymer Surfaces

As previously mentioned in conjunction with FIG. 19, all or a portion ofthe body 22 can comprise an electrically conductive polymer. Theconductivity of the polymer used preferably has a resistivity close tothe resistivity of tissue (i.e., about 500 ohm.cm). In use, theelectrically conductive body 22 can be used in association with aninterior electrode 200, like that shown in FIG. 42A. In such anarrangement, a hypertonic saline solution 204 also fills the interior ofthe electrically conductive body 22 (as also shown in FIG. 42A), toserve as an electrically conductive path to convey radio frequencyenergy from the electrode 200 to the body 22. In effect, in thisarrangement, the electrically conductive body 22 functions as a “leaky”capacitor in transmitting radio frequency energy from the interiorelectrode 200 to tissue.

Various methodologies can be used to control the application of radiofrequency energy to capacitively coupled electrode structures and toelectrode structures having electrically conductive bodies. Thepreviously described D_(50C) Function can be used, as can the previouslydescribed Duty Cycle and Temperature Disabling techniques. Withcapacitively coupled electrode structures and electrode structureshaving electrically conductive bodies, the minimal effects of convectivecooling by the blood pool enables the use of actual sensed temperatureconditions as maximum tissue temperature TMAX, instead of predictedtemperatures. Because of this, such structures also lend themselves tothe use of a proportional integral differential (PID) control technique.Illustrative PID control techniques usable in association with theseelectrode structures are disclosed in copending U.S. patent applicationSer. No. 08/266,023, filed Jun. 27, 1994, entitled “Tissue Heating andAblation Systems and Methods Using Time-Variable Set Point TemperatureCurves for Monitoring and Control.”

Various features of the invention are set forth in the following claims.

We claim:
 1. A method of ablating heart tissue, comprising: providing a collapsible ablation electrode assembly including an electrically conductive polymer body adapted to transmit electrically energy to heart tissue and adapted to selectively assume an expanded geometry having a first maximum diameter and a collapsed geometry having a second maximum diameter less than the first maximum diameter, the electrically conductive polymer body including a wall and an electrically conductive material located in the wall, and the electrically conductive polymer body having a resistivity selected to provide a lesion in the heart of sufficient depth to substantially block electric signal propagation through the heart tissue; expanding the electrically conductive polymer body so that the electrically conductive polymer body is in contact with heart tissue; ablating the heart tissue by transmitting electrical energy to the heart tissue through the electrically conductive polymer body.
 2. The method of claim 1, wherein all or at least a portion of the wall is coextruded with said electrically conductive material.
 3. The method of claim 1, wherein said electrically conductive material is a member from the group consisting of carbon black and chopped carbon fiber.
 4. The method of claim 1, further including an electrically conductive shell carried by the electrically conductive polymer body and electrically coupled thereto.
 5. The method of claim 1, wherein the electrode assembly is a first electrode assembly, and the method further includes withdrawing the first electrode and in a same or separate procedure, providing an additional collapsible ablation electrode assembly of a family of electrode assemblies, the additional electrode assembly having an electrically conductive polymer body adapted to transmit electrically energy to heart tissue and adapted to selectively assume an expanded geometry having a first maximum diameter and a collapsed geometry having a second maximum diameter less than the first maximum diameter, the electrically conductive polymer body including a wall and an amount of electrically conductive material coextruded into the wall that is different than an amount of electrically conductive material coextruded into the wall of the first electrode assembly causing the additional electrode assembly to have a resistivity different than the resistivity of the first electrode assembly; expanding the electrically conductive polymer body of the additional electrode assembly so that the electrically conductive polymer body is in contact with heart tissue; ablating the heart tissue by transmitting electrical energy to the heart tissue through the electrically conductive polymer body of the additional electrode assembly.
 6. The method of claim 5, wherein said electrically conductive material of the additional electrode assembly is a member from the group consisting of carbon black and chopped carbon fiber.
 7. The method of claim 5, further including an electrically conductive shell carried by the electrically conductive polymer body of the additional electrode assembly and electrically coupled thereto. 